The date of the Easter Sunday can be calculated using the following method derived from the original algorithm by Carl Friedrich Gauss. The algorithm can be applied to the Gregorian calendar for years 1900-2099. The date given by the algorithm is used in Western Christian churches, the date used by the Eastern churches can be 1, 4 or 5 weeks later. More comprehensive forms (including years 1583-9999) of the algorithm exist but they include lenghty tables of Epact and Sunday Letter constants so I won't add them here.
a = year mod 19 (use year in four digits)
b = year mod 4
c = year mod 7
d = (24+19a) mod 30
e = (5+2b+4c+6d) mod 7
Easter Sunday is on the (22+d+e) of March or on the (d+e-9) of April, depending on which answer is a valid date. If the date given by the algorithm is 26th of April, the Easter Sunday is on 19th of April. If the date is 25th of April and d=28 and a>10, the Easter Sunday is on 18th of April.