The calculation that fixes Easter — and the reason two churches get different answers.
Computus is the calculation that determines the date of Easter: the first Sunday after the first full moon on or after the vernal equinox. Working that out consistently occupied medieval scholarship for centuries and is the origin of a good deal of calendar mathematics.
There are two computations in use. Western churches apply the rule on the Gregorian calendar; most Orthodox churches apply it on the Julian, using a different equinox date and a different lunar table. The same rule, stated the same way, on two calendars.
The results diverge by up to five weeks. That is not an approximation or a disagreement about doctrine — it is two arithmetics, each internally exact, and each producing public holidays for different countries in the same year.
Two festivals, both defined as forty-eight days before Easter, thirty-five days apart in 2027 — because one counts from the Western Easter and the other from the Orthodox one.
| 🇦🇷 | Monday 8 February 2027Argentina | Carnival MondayEaster minus 48. Western Easter 2027 is 28 March. |
| 🇬🇷 | Monday 15 March 2027Greece | Clean MondayOrthodox Easter minus 48. Orthodox Easter 2027 is 2 May — five weeks later. |
| 🇨🇾 | Monday 15 March 2027Cyprus | Green MondayThe same offset from the same Orthodox computus, under a different name. |
Computus is the calculation. A moveable feast is any date that depends on it, or on another non-fixed rule.
Orthodox computus uses Julian reckoning, but the difference is not simply the thirteen-day calendar offset — the lunar tables differ too, so the gap varies year to year.
See also moveable feast · relative to easter · julian calendar · gregorian calendar
We hold this as rule_type: relative_to_easter and relative_to_orthodox_easter. See what we hold.