Which calendar reform made January 1 the official start of the year in many Catholic countries in 1582?

The Gregorian calendar made January 1 the official start of the year in many Catholic countries in 1582.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct a gradual drift between the calendar date and the astronomical seasons. The older Julian calendar treated the year as slightly too long, causing the date of the spring equinox to move over the centuries.

The reform changed the leap-year rule and omitted 10 calendar dates in the first countries that adopted it. Thursday, October 4, 1582, was followed by Friday, October 15. The calendar also retained January 1 as New Year's Day, a date already used in the Roman and Julian traditions.

Adoption was not simultaneous worldwide. Protestant, Orthodox, and other countries adopted the Gregorian system at different times, and some communities still use other calendars for religious or cultural purposes. This explains why historical New Year dates can differ by country.