The Gregorian Calendar is the current calendar, which has replaced the Julian Calendar. It is the most widely used calendar internationally, and believed to be more accurate to the solar year than the Julian Calendar.
The calendar takes its name from Pope Gregory XIII, who promulgated it in a Papal Bull on 24 February 1582. Despite this, many countries took centuries to adopt the calendar, and in non-western countries, it is a recent introduction. Some Protestant countries resisted it as a Popish plot.
The UK officially adopted the Gregorian Calendar on 1752.
The Russian Orthodox Church continues to celebrate its festivals according to the Julian Calendar, despite secular Russia adopting the calendar in 1918, after the Russian Revolution.