Happy Easter!

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I’m writing this the day before Easter Sunday 2025. In AD 325 at the Council of Nicaea, Easter Sunday was decided to be the first Sunday following the first full moon on or after the spring equinox – March 21st. However, the Julian calendar had a year that was too long, so by the time of Pope Gregory XIII the equinox was happening around March 11th. The Gregorian calendar fixes this issue and in the part of the world where I live (England) it was adopted in late 1752. I’ve therefore written a program in Sharp SP-5025 BASIC to that can be used to calculate the date of Easter in England(*) from 1753 onwards.

My first attempt used the EASTR program in Celestial BASIC, by Eric Burgess. However, the implementation of the O’Beirne algorithm used is faulty … it gives this year’s Easter Sunday (2025) as 13th April; and of course, Easter Sunday is tomorrow – 20th April. Digging further into his code, the algorithm has been obviously kludged as there are manual corrections for 1974, 1984 and 1994 that should not be necessary.

Instead, I’ve gone back to first principles and implemented the ’10 step’ O’Beirne algorithm from scratch. As far as I can tell, my implementation correctly calculates Easter Sunday for every year from 1753 onwards.

The dates of Easter Sunday for 2025 – 2035 inclusive. Output from the Pico MZ-80K/A running in 80K mode.

The latest Easter can fall is 25th April. The last time this happened was in 1943, the next time will be in 2038. It can also fall on 25th April in a leap year, but the first time this will happen is not until 3784!

BASIC code to calculate the date of Gregorian Easter Sunday on a Sharp MZ-80K using the SP-5025 interpreter
The code. The O’Beirne algorithm is implemented between lines 140 – 320 inclusive.

The tidy version of this code can be found along with my other MZ-80K programs.

(*) Any other country that uses the Gregorian calendar too, of course.