Easter is the only major Christian feast whose date requires an algorithm, and the algorithm is more artificial than the usual summary admits.
The rule is: the first Sunday after the first full moon on or after 21 March. Every term in that sentence needs qualifying.
21 March is a fixed ecclesiastical equinox, not the astronomical one. The astronomical equinox moves between the 19th and the 21st. The church fixed the date and does not adjust it.
The full moon is not the astronomical full moon either. It is the ecclesiastical full moon, generated by tables, the Metonic cycle of nineteen years, in which 235 lunar months approximate 19 solar years to within about two hours. Each year receives a golden number giving its place in the cycle, and from that an epact, the age of the moon at the start of the year, from which the tabular new and full moons are derived. The Gregorian reform added corrections to keep the tables from drifting.
Sunday breaks a tie: if the ecclesiastical full moon falls on a Sunday, Easter is the following Sunday, not that day. This is deliberate and ensures Easter never coincides with the tabular 14 Nisan.
The result, in Gregorian reckoning, can fall between 22 March and 25 April.
The system is a set of tables designed to be computable by anyone with a calendar and no astronomy, in an age when astronomical prediction was neither available nor uniform across a continent. Judged as an approximation intended to be portable, it is a considerable achievement.