My wife recently dug out her copy of the 1662 Book Of Common Prayer to look up the answer to this question, and it turns out the answer is much simpler than you might imagine…
“THIS Table contains so much of the Calendar as is necessary for the determining of Easter; to find which, look for the Golden Number of the year in the first Column of the Table, against which stands the day of the Paschal Full Moon; then look in the third column for the Sunday Letter, next after the day of the Full Moon, and the day of the Month standing against that Sunday Letter is Easter Day. If the Full Moon happens upon a Sunday, then (according to the first rule) the next Sunday after is Easter Day.
To find the Golden Number, or Prime, add one to the Year of our Lord, and then divide by 19; the remainder, if any, is the Golden Number; but if nothing remaineth, then 19 is the Golden Number. To find the Dominical or Sunday Letter, according to the Calendar, until the year 2099 inclusive, add to the year of our Lord its fourth part, omitting fractions; and also the number 6:
Divide the sum by 7; and there is no remainder, then A is the Sunday Letter: But if any number remaineth, then the Letter standing against that number in the small annexed Table is the Sunday Letter. For the next following Century, that is, from the year 2100 to the year 2199 inclusive, add to the current year its fourth part, and also the number 5, and then divide by 7, and proceed as in the last Rule.
Note, that in all Bissextile or Leap Years, the Letter found as above will be the Sunday Letter, from the intercalated day exclusive to the end of the year.”
Actually, calculating the date of Easter is child’s play compared to explaining exactly how this method was arrived at – which takes more than 12,000 words on Wikipedia – while still leaving most of us none the wiser.