The Council of Nicaea established that Easter would be held on the first Sunday after the first full moon occurring on or after the vernal equinox, March 21, the first day of Spring. From that point forward, the Easter date depended on the ecclesiastical approximation of March 21 for the vernal equinox.

Why does Easter's date change every year?

This means its date on the Gregorian calendar can vary every year. ... By taking into account the date of the vernal equinox (which this year fell on Saturday 20 March) and the next following full moon (Sunday 28 March), it was therefore calculated that Easter Sunday would take place on Sunday 4 April in 2021.

How is the date of Passover determined?

It begins on the 15th day of Nisan, which is the seventh month in the Jewish calendar. It ends on the 21st of Nisan in Israel (and for Reform Jews) and on the 22nd of Nisan elsewhere. Since Hebrew days begin and end at sundown, Passover begins at sundown on the preceding day.