In the Gregorian calendar, New Year's Day is the first day of the calendar year, 1 January. Most solar calendars begin the year regularly at or near the northern winter solstice, while cultures and religions that observe a lunisolar or lunar calendar celebrate their Lunar New Year at less fixed points relative to the solar year.

In ancient Roman religion and myth, Janus is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janus (Ianuarius). According to ancient Roman farmers' almanacs, Juno was mistaken as the tutelary deity of the month of January, but Juno is the tutelary deity of the month of June.