What is the calendar for?

A calendar is a system for marking dates based on time.      
The calendar allows us, on the one hand, to situate the past, and on the other hand, to organize the future. My grandchildren often express time in terms of the number of “sleeps”. Adults use a much more complicated system, the calendar, which is not even based on the decimal system. A large number of elementary units of measurement (second, minute, hour, day , week , month , quarter , semester , year , century) are at our service to give rhythm to our life, and we sometimes abuse them, for example with the expression “Wait a second”.
Nowadays,is often associated with a diary and a watch. These three tools mainly allow us to organize our social life and our activities . Even the hermit cannot do without it. Without them, we would not be able to arrive at work or to an appointment on time, call in our leave, go on vacation , cultivate our garden , celebrate birthdays , commemorate important events , etc.

History of the calendar

Even though prehistoric humans did not have a calendar, they already had a good sense of time by simply counting the days. Later, men realized that the shape of the moon changed over the days, with a cycle of approximately 30 days. They then invented months. Later still, they learned about the seasons, with a cycle of approximately 12 months, corresponding to a year. Men have also been interested in the time that passes in a day, between daybreak and nightfall. When they stuck a stick in the ground, they saw the shadow circling around the stick. It’s “the clock is ticking”. As this method could not work overnight, they had the idea of ​​making water flow, little by little, from one container to another, and thus, to see “time passing”. The expressions we use today are therefore not accidental.

Later, sundials replaced the stick stuck in the ground, around 3500 BC. Obelisks also served as sundials for the Egyptians. To measure time in the absence of the sun, at night, the Egyptians invented the clepsydra, around 1500 BC. The clepsydra is a graduated and pierced container which, filled with water, empties slowly and regularly over time.

The current calendar is the Gregorian calendar. Even the Chinese calendar, in force in China until January 1, ¹⁹²⁹ , was also replaced by the Gregorian calendar.

If the calendar allows us today to understand astronomy well, astronomy first made it possible to create the calendar!
Historically, calendars have been based either on lunations (the lunation is a period of approximately 29 and a half days during which the moon makes a complete revolution around the earth), or on the revolution of the earth around the sun, which is carried out in one year, or on a hybrid system, called lunisolar. Religions also influenced the development of calendars.   

• The first Chinese calendar was created in 2637 BC by the Yellow Emperor. The Chinese had the habit of resetting the calendar year at the start of each emperor's reign. 
• Among the Babylonians , the length of the months was based on lunations. The year was made up of 12, 13, or even 14 months, to be in accordance with the solar system. 
• The Muslim calendar is a lunar calendar. The first day of each month is the one that corresponds to the new moon: consequently, the first day of the month is not the same in each country in the world. The year has 354 days: consequently, the calendar seasons shift from one year to the next, compared to the solar seasons.
• The Hebrew calendar is used to calculate the dates of Jewish religious holidays. The day begins at nightfall: the length of a day therefore depends both on the season and the location. The month is the period of 29 or 30 days during which the moon rotates around the earth. A year has either 12 months, that is to say 354 days, or 13 months. Years of 13 months are embolismic years.   
• The Egyptians first used a lunar calendar, then they adopted a solar calendar. The Egyptian calendar year was then made up of 12 months of 30 days, plus 5 days at the end of the 12th month.
• The Greeks first adjusted the length of the months to the lunations. The year of the Greek calendar consisted of 12 months of 29 or 30 days, or 354 days, to which they added a 13th month every 2 or 3 years. In the 8th century BC, they adopted a lunisolar calendar, composed of a period of 8 years called octaeteride. To the 12 months of 29 or 30 days, they interspersed three additional months to obtain 2922 days per period of 8 years, or an average of 365.25 days per year.
• In the days of Roman calendars , the years were counted from the founding of Rome, in 753 BC. Several calendars followed one another during Roman civilization.
  • The Roman calendar of Romulus had 10 months of 30 or 31 days, from March 1 to December 30, or 304 days ^. The months of September, October, November, and December were aptly named, since at the time, they were respectively the seventh, eighth, ninth, and tenth months of the year. The year therefore began on March 1st.
    This calendar was therefore still approximate: 3 years were enough to shift the summer solstice to December, and agricultural profitability was affected due to sowing carried out at the wrong time. 
  • Around 700 BC, the Roman king Numa Pompilius added the months of January and February, to obtain a year of 12 months. Each month had 29 or 30 days, except February, which already had 28 days. The year thus had 355 days, and an additional month of 29 days was added every 4 years.   
  • Around 450 BC. the Republican Roman calendar had 12 months, plus one month every 2 years, which gave an average year of 366.25 days.    
  • In 46 BC, Julius Caesar created the Julian calendar , with the help of the astronomer Sosigenes of Alexandria. The year 46 BC was a year of 445 days, to make up for the delay linked to the previous calendar, and the first day of the year was January 1st. The Julian calendar was the reference from 45 BC to 1582 AD. It already had 12 months of 28, 30, or 31 days, with leap years.
• > The Gregorian calendar , named after Pope Gregory XIII, was decreed in 1582. It was this pope who reformed the Julian calendar of the time. This reform mainly consisted of removing 10 days from the calendar. At that time, the consequences of this decision were much less important than today, but the Gregorian calendar was accepted more or less late in different countries around the world, depending on their dominant religion. Protestants and Orthodox found it difficult to recognize the power of the Catholic Church over time. The implementation of the Gregorian calendar in the world in general, and Europe in particular, took place until the last century:   
  • For the Catholics of Rome, Spain and Portugal, the days from October 5 to October 14, 1582 never existed. Thursday October 4, 1582 of the Julian calendar is followed by Friday October 15, 1582 of the Gregorian calendar. It is for this reason that the Spanish nun Saint Teresa of Avila died on the night of October 4 to 15, 1582.
  • For the French, the day after Sunday December 9, 1582 of the Julian calendar was Monday December 20, 1582 of the Gregorian calendar.
  • In Europe, Germany adopted the Gregorian calendar in 1699, Denmark in 1700, England in 1752, Greece in 1916, and Russia in 1918. In 1699, Sweden courageously decided
    to gradually move to the Gregorian calendar from of 1700, but after some hesitation, she returned to the Julian calendar in 1711, and created February 30, 1712 to synchronize. It finally adopted the Gregorian calendar in 1753, thus deciding that the day after February 17, 1753 would be March 1, 1753.   
  • The Japanese adopted the Gregorian calendar in 1873, but the Japanese calendar is still used today for celebrating holidays.
  • In China, the Chinese calendar was replaced by the Gregorian calendar on January 1, 1929.
• In France, in 1793 (Convention of October 5, 1793), the Gregorian calendar was replaced by the Republican calendar:
  The Republican calendar had 12 months of 30 days, plus 5 or 6 days not attached to a month: Virtue, Genius, Work, Opinion, Awards, and Leap Year Revolution Day.
  Decades (periods of 10 days) were used instead of weeks, with each month therefore comprising 3 decades. The different days of the decade were primidi, duodi, tridi, quartidi, quintidi, sextidi, septidi, octidi, nonidi and decadi. The fact of abolishing Sunday and of not working one day out of ten, the decad, was variously appreciated... 
  The months were all renamed, and the saints were abandoned in favor of the names of fruits, animals, plants and 'tools.
  The proclamation of the republic, on September 22, 1792, corresponded both to an equinox day (day and night are equal... quite a symbol!), and, retroactively, to the 1st day of the year UN of the Republican calendar.
  The twelve months of the Republican calendar are as follows:
  • The spring months: Germinal, Floréal, Prairial. 
  • The summer months: Messidor, Thermidor, Fructidor.
  • The autumn months: Vendémiaire, Brumaire, Frimaire.
  • The winter months: Nivôse, Pluviôse, Ventôse. 
• In fact, the republican calendar will only be used by the state, the army, and a few revolutionaries. The main reason for this minimalist use of the republican calendar is on the one hand that the change was too important, on the other hand that the French remained attached to traditions. And why a republican calendar specific to France, when even England had adopted the Gregorian calendar.   
• On September 9, 1805, Napoleon Bonaparte deemed it useful to abandon the Republican calendar and return to the Gregorian calendar: the year 1806 therefore began again on January 1 for the French.
  Today, in our era of open borders, we can only congratulate Napoleon, at least on this point: could we imagine a European currency and a Republican calendar used in France only?     

Some units of time

• The day (in Latin: star, god or day of rest.)
   For purists, and in opposition to the night, the day is the interval of time which separates the rising and setting of the sun. If we adopt this definition, the length of a day varies on the one hand depending on the season, on the other hand depending on the location. Thus, at the North Pole and the South Pole, the duration of a day can reach 6 months!
  Brought back to the calendar, the day is a unit of time of 24 hours, or 1440 minutes, or 86,400 seconds, except:
  • on December 31, 2008, as well as December 31, 2005, where a second was added to adjust the gap between the atomic clock and the rotation of the earth. The duration of December 31, 2008 is therefore 86401 seconds! Indeed, since 1972, the duration of a second, until then based on the rotation of the earth, is now based on atomic clocks. And it is also these atomic clocks which made it possible to confirm that if the duration of a second does not vary, the duration of the rotation of the earth varies: the duration of the day lengthens by one to two milliseconds per century, due to a slowdown in the Earth's rotation.
  • Multiple expressions honor the day: Act in broad daylight, Beautiful as the day, Tomorrow it will be daytime, Give the day, Update, See the day, The big day.
• The week :
  • this period of 7 consecutive days, from Monday to Sunday, dates back to the time of the Chaldeans (11th to 6th century BC), each day being represented by one of the 7 planets known at that time: Saturn, Jupiter, Mars, Sun, Venus , Mercury, and the Moon. Uranus and Neptune were still unknown, and the Sun was a planet like the Moon.
    The correspondence between these “planets” and the days of the week is therefore as follows:
    • Monday: Moon Day
    • Tuesday: March Day
    • Wednesday: Mercury day
    • Thursday: Jupiter Day
    • Friday: Venus Day
    • Saturday: Saturn day
    • Sunday: day of the Sun
  • The week is an important period in everyday life:
    • The Christian religion sets Sunday as the day of rest, the first day of the week being Monday. For the Jewish religion, Saturday is a day of rest, and the week begins on Sunday. The Muslim religion celebrates Friday as a day of prayer and rest, and the week begins on Saturday. For displaying the weekly calendar and table calendars, you can choose the first day of the week, for example according to your religion or the customs of your country (Options, calendar layout, etc.)   
    • Labor law gives the week a particular meaning, insofar as it is the duration of weekly work which is fixed: (40 hours, 39 hours, 35 hours), with the consequence of calculating overtime.
    • The rhythm of life is often based on the week: weekly meetings, television programs, shows, organization of school programs, etc.
    • Each week has a number in the year: the week with number 1 is the one which contains the first Thursday of the year (ISO 8601 standard).
    • Direct access to the weekly calendar :
      With the weekly calendar, you can enter your calendar by clicking in a box, and enter weekly notes by clicking on the footer at the bottom of the calendar.
• The month.
  Originally, the month was the main unit of time in lunar calendars. The moon circles the earth in a period of 27 days and approximately 8 hours. As the earth revolves around the sun, in the same direction as the moon revolves around the earth, the apparent duration, for an earthling, of the revolution of the moon around the earth, is 29 days and approximately 12 hours. In fact, this period, called lunation, changes depending on the months, and even the years.
  The Gregorian calendar is a solar calendar, based on a year of 365 or 366 days, which explains why the average length of a month is greater than the average length of a lunation.
  Most months owe their name to the Roman calendar of Romulus:
  • January and February did not exist in Romulus' calendar. Their origin is therefore a little more recent. recent. January ( Januarius ) is named after the Roman god Janus. February (Februarius), dedicated to Neptune, has its origins in the Latin word 'Februare', which means 'to purify'.
  • Mars ( Martius ) owes its origin to the Roman god Mars, god of war.
  • The origin of the month of April ( Aprilis ) is uncertain. Is this month dedicated to the Greek goddess Aphrodite, or the Roman goddess Venus? Regardless, it's still the goddess of love. In Latin, Aprilis means 'Open', and April is the month when flowers open. 
  • Mai ( Maius mensis ) originated from the Roman goddess Maia.
  • June ( Junius ) owes its name to the Roman goddess Juno, wife of Jupiter, goddess of marriage. 
  • The origin of the months of July and August is also a little more recent. The name of the month of July (Julius) was given in honor of Julius Caesar, in 38 BC, and it was the Emperor Augustus who created the month of August, in the year 8 BC. If these two months are 31 days, it is because Augustus could not accept that his month had fewer days than that of Julius Caesar.
    Before these developments, July and August were respectively called Quintilis (fifth) and Sextilis (sixth), due to the fact that the year began in March.    
  • The months of September (seven), October (eight), November (nine) and December (ten) are respectively the seventh, eighth, ninth and tenth months of the Romulus calendar.
  • With monthly calendars, you can enter your calendar by clicking in a box, and enter monthly notes by clicking on the footer at the bottom of the calendar. 
    
    

    Monthly calendars
    In column	In table

• The quarter is a time unit used in education, as well as in businesses, to mark intermediate periods in the year, to make academic, financial or commercial assessments. 
  With quarterly calendars, you can enter your calendar by clicking in a box, and enter quarterly notes by clicking on the footer at the bottom of the calendar. 
   
  

  Quarterly calendars
  Civilians	School
  1st trimester	September to December
  2nd quarter	January to April
  3rd quarter	April to July
  4th trimester

  
   
• The semester allows you to take stock in the middle of a calendar year.
  With semester calendars, you can enter your calendar by clicking in a box, and enter semester notes by clicking on the footer at the bottom of the calendar.
  
  

  Semester calendars
  In column	In table
  1st semester	1st semester
  2nd semester	2nd semester

  
   
• The year did not always begin on January 1, and it did not always have 365 or 366 days.
  The year is defined as the period during which the earth rotates around the sun. Today we know that this period is equal to 365.2422 days, but our ancestors were not always so precise, and many adjustments were made throughout history:
  • The Egyptian, Greek and Roman calendars began on March 1st. 
  • With the Julian calendar, Julius Caesar started the year on January 1st.    
  • Charlemagne brought the start of the year back to March ^1st . The first Catholic kings of France decided that the year would begin at Easter, hence the tradition of gifts given on April 1st. And to put it simply, a year then consisted of eleven or thirteen months.
  • On August 9, 1564, at the Château de Roussillon, in Isère, Charles IX and his mother, Catherine de Medici, decided that the year 1565 would begin on January 1, ^by signing the Edict of Roussillon. This event is less known but more intelligent than the massacre of Saint Bartholomew, which took place under the reign of this same king, from August 24, 1572. The year 1565 is thus the first with both real gifts offered on January 1st, and with fake gifts offered on April 1st. April 1 pranks and jokes date from this year. 
  • In 1582, the Gregorian calendar was adopted by France, Italy, Spain, Portugal, Austria and Poland. Also, the following kings of France, who were undoubtedly already thinking about Europe, judged it prudent not to change the order of things.
  • In 1792, the "republic" rightly decided to change everything, including the calendar. The year then began on September 22.
  • In 1805, Napoleon Bonaparte, an emperor who was passing by, deemed it useful to return to the Gregorian calendar, which therefore began again on January ¹ .
  Nowadays, we mainly distinguish between the calendar year, which begins on January 1 ^, and the school year, which begins at the beginning of September. In companies, annual leave is sometimes counted from May or June, and the date of the accounting statement is not necessarily set at December 31. Also, the benefit of an annual calendar that does not start in January is obvious. With the sliding calendar , you can make a tailor-made calendar by giving the departure month and the arrival month.
  The Gregorian calendar is a solar calendar, based on a year whose average duration is exactly equal to the revolution of the earth around the sun, i.e. 365.2422 days. To avoid the number of days in each year being a secimal number, a year therefore has either 365 days or 366 days. 
  Non-leap years have 365 days while leap years have 366 days. A leap year is either divisible by four but not by a hundred, or divisible by four hundred. This leap year rule makes it possible to obtain an average length of the year equal to 365.2425 days, which is very close to the real length equal to 365.2422 days and corresponding to the rotation of the earth around the sun.
  It is also the rotation of the earth around the sun which explains why the length of day and night vary according to the seasons.    
  With annual calendars, you can enter your calendar by clicking in a box, and enter annual notes by clicking on the footer at the bottom of the calendar.
  
   
  

  Annual calendars

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