In 325 the Council of Nicaea set the date of Easter as the first Sunday after the first Ecclesiastical Full Moon after the vernal equinox.
The ecclesiastical (imaginary) full moon occurs on the 14th day after the new moon, and can vary up to 2 days from the real full moon.
What is perhaps most important to remember is that the determination of Easter is primarily a matter of ecclesiastical discipline and not of astronomical science.
The moon according to which Easter is calculated is not the moon in the heavens, but simply the moon of the calendar.
This calendar moon is admittedly a fiction.