Most people are only vaguely aware that Easter arrives on different days of the year. They just look at the calendar and work from there. But it really is a very simple and compact formula, though with a long and confusing history. At the Council of Nicea (325), called by Constantine I, it was decided that Easter should be celebrated on the first Sunday on or after the first full moon on or after the vernal equinox, though it wasn't stated in exactly that way. The vernal equinox is the first day of spring when the day and the night are of equal length. It is midway between the longest and shortest days of summer and winter, and opposite the autumnal equinox. The Council of Nicea tried to coordinate the solar and lunar calendars for this holiday. This year the equinox came on the 20th of March. Saturday the 31st was the full moon, so Sunday (today) is Easter Sunday. You don’t have to remember any of this, but sometimes it’s just nice to know how things work.