The early Roman calendar consisted of 10 months and 304 days, with each new year beginning at the vernal equinox; according to tradition, it was created by Romulus, the founder of Rome, in the eighth century B.C. A later king, Numa Pompilius, is credited with adding the months of Januarius and Februarius. Over the centuries, the calendar fell out of sync with the sun, and in 46 B.C. the emperor  Julius Caesar decided to solve the problem by consulting with the most prominent astronomers and mathematicians of his time. He introduced the Julian calendar, which closely resembles the more modern Gregorian calendar that most countries around the world use today.

As part of his reform, Caesar instituted January 1 as the first day of the year, partly to honor the month's namesake: Janus, the Roman god of beginnings, whose two faces allowed him to look back into the past and forward into the future. Romans celebrated by offering sacrifices to Janus, exchanging gifts with one another, decorating their homes with laurel branches and attending raucous parties. In medieval Europe, Christian leaders temporarily replaced January 1 as the first of the year with days carrying more religious significance, such as December 25 (the anniversary of Jesus' birth) and March 25 (the Feast of the Annunciation); Pope Gregory XIII reestablished January 1 as New Year's.