At the Council of Nicaea, This duty fell on this officiate because of the erudition at Alexandria he could draw on. The rules to determine this are complex, but Easter is the first Sunday after a full moon occurring after the northern vernal equinox, which falls on or after 21 March, which was its nominal date at the time of the First Council of Nicaea. Shortly after Julius Caesar reformed the calendar, the northern vernal equinox was occurring on the nominal date of 25 March. This was abandoned shortly after Nicaea, but the reason for the observed discrepancy was all but ignored (the actual tropical year is not quite equal to the Julian year of 365 days, so the date of the equinox keeps creeping back in the Julian calendar).