No, not a picnic basket, nor Hemingway’s book about Paris. ‘Movable Feast’ is the Christian church’s term for a festival whose actual date varies. Easter is such a feast, and not just because it has to fall on a Sunday, and Good Friday has to be (duh) a Friday; that would limit the variation to a week, but it’s much wider. I can’t claim to be in on the arcana of the calculations; the protestants and Roman Catholics have one way of working it out in common, and the Greek Orthodox another one entirely. As for the Old Calendarist Orthodox, no doubt they have their own way too.
Orthodox and Protestant/Catholic Easter coincide once every four years; this year Orthodox Easter is a week after Protestant/Catholic, but sometimes the difference is as much as a month. Whenever I ask anyone how Orthodox Easter is calculated, he or she says ‘Well it’s got something to do with the moon’, but they don’t say what. This year, however, I find a clue in my Greek diary: Orthodox Easter coincides with the start of the last quarter of the moon. Now when the last quarter of the moon starts, it rises at midnight. (Proper, sidereal midnight, i.e. half-way between sunset and sunrise, whatever the clock says.) And in the Orthodox tradition, the resurrection of Christ is celebrated, with much ringing of bells, fireworks, and in some parts of Greece gunshots and even sticks of dynamite tossed playfully about, on the dot of midnight between Saturday and Sunday.
Here — yes, I know I’ve put it in the blog before — is Piero della Francesca’s painting of the terrifying original event: