Easter falls on different dates each year. This is because, in the West, Easter Sunday is the first Sunday after the first full moon following the spring equinox. Apparently the equinox is deemed to occur on 21 March (although this year it actually occurred on 20 March).
In the Orthodox tradition, it’s often a later date, mainly (it seems) because it’s considered that the decision at Nicea in 325 was that Easter has to occur after the Jewish Passover. Under the Jewish calendar, the Passover is a fixed date – but the Jewish calendar isn’t synchronised with the Gregorian calendar!
The issue of the differences between the Julian and Gregorian calendars is also often mentioned, however given that the equinox is an astronomical event, I’m struggling to see the relevance of this difference.