The Gregorian calendar first took effect in most Catholic countries in 1582, with October 15 following October 4.
Pope Gregory XIII introduced the calendar to correct the gradual drift between the calendar date and the seasonal year. The Julian calendar's leap-year rule was slightly too long, causing the date of the spring equinox to move over centuries and affecting the calculation of Easter.
The reform removed ten calendar dates in the first countries to adopt it. Spain, Portugal, the Polish–Lithuanian Commonwealth, and most of Italy made the change in October 1582. Other countries adopted the system later, so the same date could have different calendar labels in different places for centuries.
The Gregorian calendar is now the civil calendar used internationally, but some churches and communities still use the Julian calendar for particular religious observances. Its New Year's Day is January 1.