New Year's Day is a festival observed in most of the world on 1 January, the first day of the year in the modern Gregorian calendar. 1 January is also New Year's Day on the Julian calendar, but this is not the same day as the Gregorian one. Whilst most solar calendars (like the Gregorian and Julian) begin the year regularly at or near the northern winter solstice, cultures that observe a lunisolar or lunar calendar celebrate their New Year (such as the Chinese New Year and the Islamic New Year) at less fixed points relative to the solar year.

In pre-Christian Rome under the Julian calendar, the day was dedicated to Janus, god of gateways and beginnings, for whom January is also named. From Roman times until the middle of the 18th century, the new year was celebrated at various stages and in various parts of Christian Europe on 25 December, on 1 March, on 25 March and on the movable feast of Easter.

In the present day, with most countries now using the Gregorian calendar as their civil calendar, 1 January according to that calendar is among the most celebrated public holidays in the world, often observed with fireworks at the stroke of midnight following New Year's Eve as the new year starts in each time zone. Other global New Year's Day traditions include making New Year's resolutions and calling one's friends and family.