It turns out that this day is more accurately Easter, or at least the more astronomically accurate Easter.*
The complicated calculation goes like this: Easter falls on the first Sunday after the first full moon on or after the vernal equinox. But the Eastern Orthodox Church, for some reason, uses the astronomical full moon and the actual equinox while, strangely, Western Christianity uses a fixed date (March 21) for the equinox, and something called the paschal full moon (which may or may not be the actual full moon), derived from a table of ecclesiastical moons.
The Eastern Orthodox system is also more logically correct. In the West, Easter sometimes precedes Passover, but the Orthodox calculations ensure that Easter always follows Passover – as the resurrection followed the last supper.
You’d think they’d all want to get together. In 1963, the Second Vatican Council suggested Easter be fixed at the second Sunday in April, if all the Christian churches could agree – but nothing happened. Then, in 1997, the World Council of Churches proposed that both East and West use the most sophisticated astronomical calculations for their joint formula – but nothing happened.
*Turns out I’m quite wrong about this! (See Mockingbird‘s comment, who clearly knows a lot more about this than I do.)