Calculating Easter

Easter comes pretty late this year -- April 20th. Seems almost arbitrary. Ever wondered how they determine when Easter comes?

Well, as it turns out, it is almost arbitrary. Not quite, but ...

The first number to keep in mind is the first day of Spring, the vernal equinox. That's March 21. Sometime between March 21 and April 25th will be the official Easter. (By the way, the Eastern Orthodox Church doesn't necessarily celebrate on the same day as the rest.) So the next thing you need to determine is when the full moon occurs. Find the first full moon after the vernal equinox. Then find the first Sunday after that first full moon, and you have Easter¹! (Note: If the first full moon falls on a Sunday, then Easter will be the next Sunday.)

Why? Well, it's supposed to coincide with Passover. And this year Passover will start on Tuesday, the 15th of April and will continue for 7 days until Monday, the 21st of April. There you go. All clear. Right? (Don't blame me. I didn't set it up.) Glad to be of service.
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¹ To be accurate, it may not exactly be the actual physical full moon. There is what is known as an "ecclesiastical" full moon (or "paschal full moon"), a date set by the early Church back at the Council of Nicea (325 AD), and may not exactly coincide with the astronomical full moon.