I have a friend at work who asked my why the Coptic Orthodox Resurrection Feast is different than the Americans' which is before us by one week. My friend is interested to compare the theology of the different churches and figure out which is the right one.
The Oriental and Eastern Orthodox Churches follow the Jewish lunar system and celebrate the Holy Resurrection Feast after the Jewish Passover. The Catholic and Protestant Churches use a system by which the Resurrection Feast is celebrated on the first Sunday following the first full moon after the vernal [spring] equinox.
Your friends will benefit much more by learning and reading deeper about our Church: