How Is the Date of Easter Calculated? The Computus, Explained

Christmas is easy. It lands on December 25th every year, and you can circle it on any calendar the moment the new one arrives. Easter is the restless one. One year it falls in late March; a few years later you're hunting for it in the third week of April. It never seems to sit still — and there's a beautiful, very old reason why.

The one rule that governs everything

Here it is, in a single sentence: Easter is the first Sunday after the first full moon that falls on or after the March equinox.

That's it. Read it slowly and you'll notice it braids together three different clocks — the sun (the equinox), the moon (the full moon), and the week (Sunday). Christmas only has to satisfy the calendar. Easter has to satisfy the sky.

Because a full moon and a Sunday rarely line up neatly, the date slides. If the first spring full moon happens to fall on a Saturday, Easter is the very next morning. If it falls on a Sunday, you wait a whole week — Easter can't be the full moon; it must come after it. That single "wait for the next Sunday" is why the date can jump by more than a month from year to year.

Why the moon at all?

Because the first Easter was tied to Passover, and Passover is reckoned by the moon. The Gospels place the crucifixion and resurrection during that festival, so the early church anchored its most important day to the same lunar rhythm. When the sun and moon disagree — as a solar calendar and a lunar festival always will — you get movement. Easter inherited the moon's wandering.

The Computus: doing the math before computers

For centuries, working out this date each year was one of the most important calculations in Christendom. It even had a name: the Computus, Latin simply for "computation." Getting it right mattered enormously — whole regions could end up celebrating Easter on different Sundays, which was seen as a genuine scandal.

So medieval monks became, in effect, astronomers and mathematicians. They didn't watch the sky each spring and squint at the actual moon; that would have been chaos. Instead they used tables — a repeating 19-year lunar cycle (the moon's phases nearly repeat every 19 years) combined with the day of the week — to predict the "ecclesiastical" full moon and the equinox, which is fixed for the purpose of the calculation at March 21st. The Computus is a piece of pen-and-paper arithmetic that let a monk in a cold scriptorium know the date of Easter for years to come, without ever looking up.

It's one of the oldest algorithms we still use. The mathematician Carl Friedrich Gauss was so taken with it that he wrote out a formula for it in 1800, and versions of that formula still quietly run the calculation today.

Why Orthodox Easter is often on a different day

You may notice that Orthodox Christians frequently celebrate Easter — or Pascha — a week or several weeks later than Western churches. Both are following the same rule. The difference is the calendar underneath it: most Orthodox churches still calculate using the older Julian calendar, while the West switched to the Gregorian calendar centuries ago. The rule is shared; the ruler measuring it is not. Some years the two even coincide, which is always a quiet joy.

A gentle note

You don't need to hold any of this in your head. It's simply lovely to know that the day is decided by the sun and the moon rather than by us — that Easter arrives on its own terms, and always has.

Bible Clock keeps this arithmetic running quietly in the background. It computes the equinox, the paschal full moon, and the Sundays that follow, and then it just tells you — marking Lent, Easter, Pentecost, and the seasons of the church year on your clock, so you never have to look them up. The calculation the monks labored over is now a small, calm line on your screen.

Bible Clock quietly keeps the church calendar for you — on your iPhone, iPad, Mac, and Apple Watch. One purchase, no subscription.