Happy Easter from the 13th century! This table shows the movable feasts from 1273 to 1396, all carefully calculated by a monk at Cerne Abbey in Dorset. But the column for Easter Sunday itself is blank. Why is that? 1/5

This table uses a complex system of epacts & concurrents to find the framework of the Christian calendar. (If you want to know why Easter was a movable feast at all & people needed to know the date in advance, see this excellent thread by @j_t_palmer):
https://twitter.com/j_t_palmer/status/1248238136049664000 2/5

Spread over four pages, this table gives them all they need. The year is on the far left in Roman numerals - the first line on this page is 1368. The column of letters on the right tells you when all the Sundays that year are (the lines with two letters are leap years). 3/5

But by 1368, almost a century after the table was drawn up, the system was outdated. The medieval monks had upgraded their method of calculating the calendar. Now they used so-called Golden Numbers. (These were so effective that legends sprouted up about their invention.) 4/5

That's why no-one bothered to calculate the column for Easter Sunday -- or the age of the Moon that day, on the far right -- for any year after 1334. Monks had busy lives! But luckily for us, they kept and carefully stored the manuscript for another 200 years. 5/5