Since the 16th century. In the Middle Ages, the year began on Easter Day. But Easter fluctuated according to the moon. Pope Gregory XIII modified the Julian calendar and had the Gregorian calendar adopted, which removed 10 days in one fell swoop. Thus, Friday October 15, 1582 succeeded Thursday October 4, 1582. Explanatory notes.
The New Year hasn’t always been celebrated on January 1. In ancient Rome, the lunar calendar consisted of ten months and 355 days. The year began in March. It was Julius Caesar who, in 46 BC, decided to adopt a 12-month solar calendar. The aim was to realign the beginning of the Roman year with the tropical year. The Julian calendar gave the months of September, October, November and December names corresponding to their rank (seven, eight, nine, ten).
However, advances in astronomy during the Middle Ages showed that there was an increasing discrepancy between the Julian calendar and the solar year. Pope Gregory XIII asked a certain Christophorus Clavius, a renowned German mathematician and astronomer, to redo the calculations.
10 days deleted
This new Gregorian calendar was officially adopted by the papal bull “Inter gravissimas” on February 24, 1582, and imposed in the Catholic states a few months later.
The Gregorian calendar retains most of the structure of the Julian calendar, ie the subdivision of the year into 12 months of 4 weeks, counting from the year of Christ’s birth. But it differs from the previous calendar in that leap years (as in 2020) regularly correct the calendar’s shift on the dates of the equinoxes.
But calculations by Christophorus Clavius showed that, over the centuries, Julius Caesar’s calendar had fallen a little behind. And that nine leap years had been counted too many.
With the stroke of a pen, Pope Gregory erased ten days from the new calendar that bears his name. Thus, from Thursday, October 4, 1582 to Friday, October 15, 1582.
From Easter to the New Year
Until the end of the Middle Ages, the year began on Easter Day throughout Christendom. But the date varied from year to year. In 325, the Council of Nicaea set Easter on the first Sunday after the full moon following the vernal equinox. In other words, between March 22 and April 25.
This makes it extremely difficult for historians to calculate the dates of certain events.
In 1564, the Edict of Roussillon established January 1st as the beginning of the calendar year. On January 1, 1622, this date was applied in all Catholic countries at the request of the Holy See.
The Gregorian calendar has gradually gained worldwide acceptance, although there are many other civil and religious calendars. The Chinese New Year (between January 21 and February 20), the Hebrew calendar (September or October).
But January 1ᵉʳ is now the first day of the year worldwide.