Easter changes every year because its date is determined as the first Sunday after the first full moon that occurs on or after the spring equinox (fixed by the Church as March 21). This method roots in the lunar calendar, linked to the Jewish Passover, and was established by the Council of Nicaea in 325 AD to unify the celebration of Jesus Christ's resurrection. Because the lunar calendar and the solar calendar do not align perfectly, the date of Easter varies between March 22 and April 25 each year.