January 1 marks the start of the New Year in the Gregorian calendar used for most international civil dates.
The Gregorian calendar places New Year's Day on January 1. It was introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar, chiefly to correct a gradual mismatch between the calendar date and the seasonal year.
The reform changed the leap-year rule: century years are not leap years unless divisible by 400. This makes the Gregorian calendar more accurate than the Julian calendar over long periods. Adoption happened at different times in different countries, so January 1 did not become a universal civil New Year date simultaneously.
Several cultures and religions still observe other New Year dates, including Lunar New Year, Rosh Hashanah, Nowruz, and New Year celebrations based on the Islamic calendar. Thus January 1 is the standard civil date in much of the world, not the only New Year observance worldwide.