Because it depends partly on the phase of the moon, the date of Easter Sunday can vary by as much as 35 days.

In 1928, the Parliament of the United Kingdom passed an act that would establish its date as the Sunday following the second Saturday in April (and thus always falling between April 9 and 15).

Though passed and enacted, the act has never come into force because the British government agreed that the change would need to be made in consultation with church leaders. It’s known as the Easter Act 1928.

(In the meantime, here’s a mnemonic for remembering the current system.)