Nineteen civil calendar years are divided into 235 lunar months of 30 and 29 days each.
This period of 19 years (the ) is used because it produces a set of civil calendar dates for the ecclesiastical moons that repeats every nineteen years while still providing a reasonable approximation to the astronomical facts.
metonic cycle
The first day of each of these lunar months is the . Exactly one ecclesiastical new moon in each year falls on a date between March 8 and April 5, both inclusive. This begins the paschal lunar month for that year, and thirteen days later (that is, between March 21 and April 18, both inclusive) is the paschal full moon.
ecclesiastical new moon
Easter is the Sunday following the paschal full moon.