For many centuries, Catholic Easter was defined in accordance with the Easter created in Alexandria. It was based on the nineteen-year cycle of the Sun, the day of the vernal equinox in it was also unchanged - March 21. And this state of affairs existed until the 16th century, until the priest Christopher Clavius ​​proposed another calendar for determining Easter. Pope Gregory XIII approved it, and in 1582 Catholics switched to a new - Gregorian - calendar. The Eastern Church refused the innovation - Orthodox Christians have everything as before, in accordance with the Julian calendar.

They decided to switch to a new style of chronology in Russia only after the revolution, in 1918, and then only at the state level. Thus, for more than four centuries, the Orthodox and Catholic Churches have been celebrating Easter at different times. It so happens that they coincide and the celebration is celebrated on the same day, but this rarely happens (for example, a similar coincidence of Catholic and Orthodox Easter was only recently - in 2017).