[aetherson]

I think that they're suggesting that this would be conventionally the case: "Okay, we're now deciding that Easter Sunday will be April 15, which is always a Sunday."

I think this is on the basis that it's important for Easter to be on a Sunday, and not nearly as important that it be the Sunday following the first full moon following the equinox, or whatever: just that once you say "it's got to be on a Sunday," you need some damn way to figure out which Sunday each year.