Since when does the year begin on January 1ᵉʳ?
Since the 16ᵉ 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.