The 1750 #UKLaw rule for calculating the date of #Easter breaks down quite simply.
The New Testament account is that Jesus rose from the dead on the day after the Sabbath, i.e. the Sunday (the Sabbath being Saturday), that followed Passover.
So what's the date of #Passover? It's the fourteenth day of Nisan, which is the first (ecclesiastical) month of the Hebrew luni-solar calendar.
Hebrew calendar months (nominally) begin at the new moon, so the 14th day is (approximately) the time of the full moon.
So when does #Nisan start? It has all been done with tables for about 16 centuries, but the underlying aim was to tie it to the vernal equinox.
Hence expressing all of this with the religions taken out: Easter is the first Sunday after the first full moon after the vernal equinox.
And because the Gregorian Calendar aimed to stabilize the dates of the equinoxes, taking even some of the #astronomy out of the rule gets this last part turned into "after the 21st of March".

