The Orthodox calendar, also known as the Julian calendar, is the calendar used by many Eastern Orthodox churches around the world to determine the dates of religious holidays and observances.
The Orthodox calendar has different dates for Easter and other religious holidays, which are based on the liturgical calendar.
This difference is due to the fact that the Orthodox churches did not adopt the Gregorian calendar, which was introduced in 1582 by Pope Gregory XIII as a reform of the Julian calendar.
The Orthodox calendar follows the same month names as the Julian calendar, which are: January, February, March, April, May, June, July, August, September, October, November and December.
The Gregorian calendar has a leap year rule that eliminates the accumulated error of the Julian calendar, so the Orthodox church still use the Julian calendar.