Why do the dates of Good Friday and Easter Sunday differ every year?
The date of the Easter weekend is determined by the date of the Jewish Passover feast. The calendar of the Jews originally worked according to the moon, and therefore it differs from ours. Nowadays Easter (or the Passover) is celebrated on the first Sunday after the first full moon after March 21. The date will therefore be determined by the moon during that time of the year.
We can be glad that it is not our duty to decide such difficult dates. Our duty is simply to believe in Jesus Christ and to obey him as our Saviour and King (1 John 5:13).