Why Today Isn't Pascha (ie, Orthodox Easter)*

(*-except in Finland!)

This brief article says it's basically because since Catholics and Protestants adopted the Gregorian calendar, they compute the Spring Equinox differently than we all did previously.

There's a movement for a common "Easter" among Christians ecumenically, but since God keeps an annual date with the Orthodox Patriarch of Jerusalem, I'd oppose any change in the date of (our) Pascha! :)