Why the Date of Easter Changes Every Year
The date Christians celebrate Easter around the world varies, and is moveable. In Western Christianity, it can fall anywhere from March 22 to April 25. Before 325AD at the Council of Nicaea, Easter was celebrated on the Sunday immediately following the first full moon after the spring equinox.
Earliest believers in Asia Minor wished to keep the observance correlated to be after the Jewish Passover – Jesus’ “Last Supper”.
It is also impacted by the Western Christian Church’s establishment of a table of Ecclesiastical Full Moon dates. These dates determine all Holy Days on the Ecclesiastical calendar. Therefore, using the Gregorian calendar, in Western Christianity Easter is always celebrated on the Sunday immediately following the “Pascal Fool Moon”, being the first full moon after March 20.
While Eastern Orthodox Christianity follow the Julian calendar and work with the lunar calendar. Easter for the Orthodox Christian Church, adhering to tradition that it must always fall after Jewish Passover, Easter cannot be celebrated before April 3 (present day Gregorian calendar), which was March 21 in 325AD.
The attached article provides further explanation, plus an Easter calendar for both Western and Eastern Christianity through to 2026 -> https://www.learnreligions.com/when-is-easter-p2-700776