My understanding is that it's a comemmeration, not an anniversary, and it's calculated something the same as Passover, but if it happens to fall on the same day as Passover they move it according to a supplemental calculation.

I'm also told that "Easter" comes from "Eostre," a Saxon goddess, and the month of April was something like "Eostremonath."

For some reason I really like things like this.

clint