Easter is a festival celebrated all over the world by the Christians but the date of Easter changes every year according to the Gregorian calendar which is a calendar accepted all over the world for civil use. Gregorian ecclesiastical rules determine the date by going back to 325 CE to the first council of Nicaea convened by the Constantine, the roman emperor use the Julian calendar during that period. They decided that Easter should be kept on Sunday which was accepted all over the world. Special tables were made for the calculation of Easter date which was revised in the coming centuries. After that many calculations were made in different ways by the Christian community in the world.
Now the reconstruction of Julian calendar was completed by Gregor XIII in 1582 which has given us new tables for Easter calculation. The Gregorian calendar was slowly accepted by the world. Still the eastern churches calculate the date of Easter in accordance to the old Julian calendar. The main difference between these two calendars is the â€œleap year ruleâ€.
The generally used statement is that Easter falls on the first Sunday after the vernal equinox, 21st march. That is why it changes its date and can fall between the March 22 and April 25.
In 1923 another modified version of Gregorian calendar came and it was decided that Easter date will be calculated according to astronomical full moon for the Jerusalem meridian. But eastern Christian churches use different-different methods and tables for this calculation.