Ostara/Easter is one of those interesting Pagan/Christian fusion holidays. Like Jul, it used to be pegged to a position in the wheel of the year: Spring Equinox.
When Ostara got syncretized into xtianity, suddenly Catholic theologians needed the feast to be pegged to Passover so that the story of the holy week works out.
photo: Apatak@wikipedia
Easy, just use the Jewish calendar, right?
No, these're medieval antisemites.
Instead, some Latinate eggheads made the Computus Paschalis, which is not a WH40K thing, but a wicked complex algorithm for the date of Easter.
As the Roman Empire fell apart, local clergy were to use the algorithm to get the date without a proclamation from Rome.
Full circle with the pagan German origin of Easter, here's an old Swedish chart—in runes!
scan: Johan Gustaf Liljegren
Jack Graham