When is Easter 2011?

If you want to calculate when Easter 2011 is celebrated, you'll need to look to the heavens – kind of.

Easter Sunday falls on a different date every year. It is determined by the phases of the moon and the liturgical calendar of the Roman Catholic Church, which considers it a "moveable feast."

In most Christian religions, Easter celebrates the resurrection of Jesus Christ, which they say occurred three days after his crucifixion (marked on Good Friday each year). In the Catholic Church, it marks the end of Lent, a 40-day (not counting Sundays) season of repentance, prayer and fasting that begins on Ash Wednesday.

So when is Easter 2011? The quick answer for most Christian religions is April 24.

Most churches determine the date by the appearance of the "paschal" full moon, which is the first full moon that rises on or after the Vernal, or Spring, Equinox. Easter Sunday comes on the first Sunday after that moon. The roots of the word "paschal" trace to the Greeks "Pascha," which itself derives from the Hebrew "pesach." In English, both words mean "Passover," which is the Jewish holy season that celebrates the Israelites' release from slavery in ancient Egypt.

After much controversy during the early centuries of Christianity over determining exactly when Easter is, this method has been widely accepted over the years.

However, determining which date Easter 2011 is on gets a little more complicated when you consider that the paschal moon is not necessarily an actual full moon and that not all religions track time by the same calendar that's hanging on your wall.

The paschal moon is one of the "ecclesiastical moons" of the lunar calendar, which is based on the lunar cycle with months of 30 or 29 days. The ecclesiastical moon falls on the 14th day of the lunar month and is sometimes is a few days off from the actual full moon because of cumulative small variances in the lunar calendar. In 2011, the paschal moon happens to fall on the same date as the full moon in the sky – April 17.

Further complicating matters is that most Eastern Orthodox churches calculate when Easter is based on the old Julian calendar, rather than the Gregorian calendar used by Western churches. Because of this, Orthodox Easter Sunday can be take place several weeks from the Western celebration. In 2011, though, Orthodox Easter is also on April 24.

Incidentally, Easter Sunday in the Western religions can fall on any date from March 22 to April 25. And as far as the Easter Bunny, don't worry; he very carefully keeps track of when Easter is, and has not missed one yet.