Further, on the basis of ancient church tables, the date of Easter was calculated
How to calculate the date of Easter
Easter was celebrated by Catholics and Orthodox on one day until 1583. Then the Roman Catholic Church switched to the Gregorian Easter, it was considered more precisely astronomically and the second point was removed from the rules. Then the Easter calendar has completely changed, and the Catholic Easter can now coincide with the Easter of the Jew. Due to the fact that the difference between the Julian and Gregorian solar calendar is 13 days and when calculating, different methods of calculating full moon are used, now Catholic and Orthodox Easter differ in the date of celebration sometimes for almost a month.