How do we date Easter?
Passover - Pesach
The Jewish calendar is lunar. Twelve lunar months, with an occasional extra month popped in to keep up with the solar year. The month begins with new moon, and full moon, in the middle of the month, is the obvious time for extensive parties and festivals. There’s more light at night! Passover (Pesach) is the first full moon after the vernal (Northern Spring) equinox (14 of Abib in the Old Testament's Hebrew Calendar) (Lev 23:5). This was to be a "perpetual ordinance" (Exodus 12:14).
Nicaea on Easter
There appears confusion between the synoptics (Matthew, Mark, and Luke) and John on the relationship between the Passover celebration and Christ’s death. Added to that, some early Christians celebrated Pascha (Easter) on the Jewish festival of Passover, whilst others always celebrated it on the Sunday following. The former were called Quartodecimans (Latin: quarta decima, fourteen). The Council of Nicaea (325) decided against the Quartodecimans and in favour of Easter always being on a Sunday. Rather than produce a canon on this, they communicated this to the different dioceses and gave the Bishop of Alexandria the privilege of announcing annually the date of Easter. The Council of Nicaea determined that Easter would be the Sunday which follows the first full moon after the vernal equinox. If full moon happens to fall on Sunday, Easter is celebrated the following Sunday. Furthermore, it fixed the vernal equinox to be 21 March. By the sixth century complex mathematical methods had been devised, involving paschal cycles of 19 years in the East, and 84 years in the West. Hence Easter calculations are based not on the astronomical full moon but an "ecclesiastical moon," based on these created tables.
Easter and the Holy Eucharist by Fr. John A. Hardon, S.J. (here)