Easter day is approaching, and this year will fall on the same day for the Eastern and Western churches (due to the adoption of the Gregorian calendar in the West in AD 1582, which was not adopted by Eastern churches who still calculate feast days using the older Julian calendar, the dates for Easter are often determined to be on different Sundays by the Eastern and Western churches).

The calculation of the date of Easter is geared to the March equinox. This is because Easter is related to Passover, or the Feast of Unleavened Bread, which Christ was celebrating with his disciples at the Last Supper. The timing of Passover was ordained in Exodus 12:1-2 and is based on a lunar calendar rather than a solar calendar.

The Explanatory Supplement to the Astronomical Almanac for 2005, edited by P. Kenneth Seidelmann of the United States Naval Observatory, explains the calculation of the date of Easter:

In the Gregorian calendar, the date of Easter is defined to occur on the Sunday following the ecclesiastical Full Moon that falls on or next after March 21. This should not be confused with the popular notion that Easter is the first Sunday after the first Full Moon following the vernal equinox. In the first place, the vernal equinox does not necessarily occur on March 21. In addition, the ecclesiastical Full Moon is not the same as the astronomical Full Moon -- it is based on tables that do not take into account the full complexity of lunar motion. As a result, the date of an ecclesiastical Full Moon may differ from that of the true Full Moon. However, the Gregorian system of leap years and lunar tables does prevent progressive departure of the tabulated data from the astronomical phenomena.

This year, the first ecclesiastical full moon after March 21 was that of April 17, which means that Easter will be this Sunday, April 24. The latest that Easter can possibly fall is April 25, which will occur in 2038, as explained in this article from Space.com by meteorologist and columnist Joe Rao. It also explains that, due to the fact that the obliquity of the ecliptic is steepest with respect to the horizon in the northern hemisphere at the spring equinox, the moon rises considerably later each night at this time (the sun, moon and planets travel on or near the ecliptic, which is the plane of the solar system -- the moon's path is not exactly on the ecliptic, or else we would have an eclipse every month).

If the concept of equinoxes or the ecliptic are confusing to you, they are detailed at great length and with numerous diagrams in the Mathisen Corollary.

The distinction between ecclesiastical full moons and astronomical full moons is not dealt with, because the celestial phenomena of greatest concern in the Mathisen Corollary

are those which were encoded by very ancient civilizations in their mythologies and monuments (primarily dealing with solstices, equinoxes, and the phenomenon of precession). However, as the calculations regarding the date of Easter demonstrate, the importance of these heavenly phenomena continues to this day.