Page "Easter" Paragraph 51

Easter is determined on the basis of lunisolar cycles.

The lunar year consists of 30-day and 29-day lunar months, generally alternating, with an embolismic month added periodically to bring the lunar cycle into line with the solar cycle.

In each solar year ( 1 January to 31 December inclusive ), the lunar month beginning with an ecclesiastical new moon falling in the 29-day period from 8 March to 5 April inclusive is designated as the paschal lunar month for that year.

Easter is the third Sunday in the paschal lunar month, or, in other words, the Sunday after the paschal lunar month's 14th day.

The 14th of the paschal lunar month is designated by convention as the Paschal full moon, although the 14th of the lunar month may differ from the date of the astronomical full moon by up to two days.

Since the ecclesiastical new moon falls on a date from 8 March to 5 April inclusive, the paschal full moon ( the 14th of that lunar month ) must fall on a date from 21 March to 18 April inclusive.