This year the Lenten season begins with the marking of Ash Wednesday on March 2, but it occurred on February 17 last year and will be on February 22 next year. How does one determine when is Ash Wednesday? For most people, it is simply a matter of looking on a calendar. But how does the church determine the date of Ash Wednesday each year? The somewhat simple answer is that it takes place 46 days before Easter Sunday. Okay, but how is the date of Easter Sunday determined?

Easter falls on the first Sunday after the first full moon in the season of Spring in the northern hemisphere. The first process is to determine the start of Spring. This is, of course, on the vernal equinox, the date when the sun moves north across the celestial equator; the day the amount of measured daylight and darkness is equal during the 24-hour period. Astronomically the vernal equinox can fall on March 19, 20, or 21 because the earth does not take exactly 365 days to complete a revolution around the sun. To simplify matters a bit, ecclesiastical rules fix the date of the equinox to March 21.

With “the first day of Spring” determined, one must find the first full moon to take place after that date. The moon is full about every 29.53 days. If the moon is full on March 21, that marks the first full moon of the Spring season. If the date happens to be a Saturday, the next Sunday is the very next day, March 22. This marks the earliest possible Easter Sunday.

If the moon is full on March 20, which is before the vernal equinox, the next phase of the full moon would occur on April 18. If that date happens to fall on a Sunday, the first Sunday after the full moon would occur on April 25. This marks the latest possible Easter Sunday.

To determine the date of Ash Wednesday, one would count 46 days before the date of Easter Sunday. But why 46 days; is the season of Lent not 40 days? Technically, Lent does last 40 days, symbolic of the 40 days Jesus spent fasting and praying in the desert prior to the start of his ministry — according to the synoptic gospels. The 40 days are symbolic of the gospel timing, but since 1970 Lent has officially been the days that fall between Ash Wednesday and Easter Sunday, less each Sunday in that time-frame. Through Christian history, this timing has varied slightly. David Philippart, the liturgy director at Old Saint Patrick’s Church in downtown Chicago, offers a complete analysis of this history.

This year is a somewhat late start to the Lenten season; the earliest possible Ash Wednesday is February 4 (which will next occur in 2285) and the latest is Mach 10 (will happen next in 2038).

A somewhat obvious question might be why Easter is not on a fixed calendar date, like Christmas. Easter does have a stated date in scripture; it happened on the Sunday after Passover. The problem being that Passover is determined on a lunar-based calendar (hence the importance of the full moon), that the Jewish people followed, but we now use a solar-based calendar, as the Romans were using in the time of Jesus. Therefore, there is not an exact correlation in our Gregorian calendar to the date of the “first Easter.” The vernal equinox can be firmly determined, without regard to the type of calendar being used. Therefore, this set date is established to calculate the date of Easter and Ash Wednesday every year.