Why isn't Easter on the same date in the Eastern and Western Church? Why doesn't Easter always coincide with Passover?

The dating of Easter is pretty complicated. The Nicene Council (ended August 25, 325 A.D.) decreed Easter to be the first Sunday after the full moon following the Spring Equinox, March 21, unless that full moon fell on a Sunday (in which case Easter would be the following Sunday). In 2008, Easter is March 23.

Learn more about the dating of Easter in: Dating Easter.