Up to 1582 all Christianity followed the Julian calendar in celebrating Easter which is the time Christians believe Christ who was crucified and died on Good Friday came back to life. In that year Pope Gregory decided that the calendar had become out of kilter with the spring equinox so he moved it back 10 days. Most of the world follows what is known as the Gregorian calendar. However the Eastern Orthodox churches decided to stick with the Julian calendar.
Easter is figured somewhat like Passover. In the churches it comes on the first Sunday after the first full moon after the spring equinox. Since the equinox in the Julian and Gregorian calendar differ, often Easter will be on different Sundays. I’m not exactly sure how Passover is figure so I’m not going to venture into explaining it. I tried to read up on it but after reading this article I became totally confused.