The Gregorian calendar first took effect in 1582.
Pope Gregory XIII introduced the calendar reform to correct a drift between the calendar year 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 first countries to adopt the Gregorian calendar included Catholic states such as Italy, Spain, Portugal, and Poland-Lithuania. In those places, Thursday, October 4, 1582, was followed by Friday, October 15, removing ten calendar dates.
Adoption was gradual rather than universal. Britain and its colonies changed in 1752, while Russia adopted the calendar after the 1917 Revolution. The Gregorian system is now the most widely used civil calendar, and it places New Year's Day on January 1.