Math in the Calendar
First, take the value of the year (for example, 1492), add one (1493), and divide the sum by nineteen (78, with a remainder of eleven; eleven is the Golden Number for the year 1492).GN=(y+1)/19
Remainder = GN
Now take the year and increase it by one-quarter of itself (373), ignoring any remainder, and divide the sum (1865) by seven (266 remainder three). Subtract the remainder from three (zero). If the difference is zero or less, subtract the remainder instead from ten (7, meaning that the Dominical Letter for 1492 is G, the seventh letter).DL=(y+1/4y)/7
Subtract remainder from 3
If the difference is 0 or less, subtract the remainder instead from 10.
Consulting the calendar in a Book of Hours, find the appropriate Golden Number between March 1 and April 12 (there are two "elevens": the first is on March 3, which, as we will see, will produce a date too early for Easter between March 22 and April 25; the second date, April 2, is the one we must use). Next, count forward fourteen days, inclusively, to the first full moon after the vernal equinox (April 15), and find the next occurrence of the Dominical Letter G (April 22). This is the date of Easter Sunday in that year. Note that if it happens to be a leap year (as was 1492), the days in January and February revert back one Dominical Letter (so that the Sundays fall on F) before "leaping" forward again (to G) in March, so as not to effect Easter. Luckily, this is only figured one time a year.
In the Roman system, the days are counted down backwards inclusively from the nones (either the fifth or seventh day) of the month, again from the ides (either the thirteenth or fifteenth day), and again from the first day of the following month (the kalends). These fixed days are indicated in the fourth column. Whereas the familiar "Ides of March" fall on the fifteenth day (as in May, July, and October), in the other months the ides come on the thirteenth. Similarly, the nones of March, May, July, and October fall nine days before ides (that is, on the seventh day), but for the other months they arrive on the fifth. According to this system, instead of January 11, the medieval person would have written "iii idus Januarii," that is, the third day counted back inclusively from what we call the thirteenth of the month. By this reckoning, it is easy to see why one might have preferred to call the sixteenth day before the Kalends of April "St. Patrick's Day."
This page is a project by Deanna