Calculating the Date of Easter

Book of Common Prayer

We refer to the year number as y, and use it to calculate the Golden number, g:

g = y mod 19 + 1

s = (y - 1600) div 100 - (y - 1600) div 400

l = (((y - 1400) div 100) × 8) div 25

p' = (3 - 11g + s - l) mod 30

if (p' == 29) or (p' == 28 and g > 11) then

p = p' - 1

else

p = p'

d = (y + (y div 4) - (y div 100) + (y div 400)) mod 7

d' = (8 - d) mod 7

p'' = (80 + p) mod 7 = (3 + p) mod 7

x' = d' - p'' = (8 - d) mod 7 - (3 + p) mod 7 = (8 - d - (3 + p)) mod 7 = (5 - d - p)) mod 7

x = (x' - 1) mod 7 + 1 = (4 - d - p)) mod 7 + 1

We can now calculate e, the number of days Easter falls after 21st March:

e = p + x

e = p + 1 + (4 - d - p) mod 7

if e < 11 then

(e + 21) March

else

(e - 10) April