The first was mainly concerned with the lawfulness of celebrating Easter on a weekday.
Second phase
The second stage in the Easter controversy centres round the Council of Nicaea (A.D. 325). Granted that the great Easter festival was always to be held on a Sunday, and was not to coincide with a particular phase of the moon, which might occur on any day of the week, a new dispute arose as to the determination of the Sunday itself.
Third phase
It was to the divergent cycles which Rome had successively adopted and rejected in its attempt to determine Easter more accurately that the third stage in the paschal controversy was mainly due.
What is perhaps most important to remember, both in the solution adopted in 525 and in that officially put forward at the time of the reform of the Calendar by Gregory XIII, is this, that the Church throughout held that the determination of Easter was primarily a matter of ecclesiastical discipline and not of astronomical science.
As Professor De Morgan long ago clearly recognized, the moon according to which Easter is calculated is not the moon in the heavens nor even the mean moon, i.e. a moon traveling with the average motion of the real moon, but simply the moon of the calendar. This calendar moon is admittedly a fiction, though it departs very little from the actual astronomical facts; but in following the simple rule given for the dependence of Easter upon the moon of the calendar, uniformity is secured for all countries of the world.
According to this rule, Easter Sunday is the first Sunday which occurs after the first full moon (or more accurately after the first fourteenth day of the moon) following the 21st of March. As a result, the earliest possible date of Easter is 22 March, the latest 25 April.