Which pope introduced the Gregorian calendar, now used for January 1 New Year’s Day?

Pope Gregory XIII introduced the Gregorian calendar in 1582, including its January 1 New Year’s Day convention.

The calendar reform corrected a small but accumulating error in the Julian calendar’s calculation of the solar year. Under the older system, the date of the spring equinox had drifted, affecting the calculation of Easter and other religious dates.

The reform skipped ten calendar dates in October 1582 and changed the leap-year rule. Century years would no longer be leap years unless divisible by 400, which is why 1600 and 2000 were leap years but 1700, 1800, and 1900 were not.

Catholic countries adopted the system first, while other countries changed later. Britain and its colonies adopted it in 1752, and Russia adopted it after the 1917 Revolution. January 1 had already been used as New Year’s Day in parts of Europe, but the Gregorian system standardized it across the reformed calendar.