復活節問題

Sorry to answer in English

Easter is not exactly base on a western calendar (which track motion of the sun), but based on a lunar calendar, similar to the Chinese calendar.

Easter is define as the Sunday after the first full momo on or after the day of the vernal equinox. Chinese Calendar called this day "春分"

Vernal Equinox is March 21, It is the day where the length of day equal length of night.