When is Easter this year? For those of you looking for a quick answer, it is March 31, 2013.   Now for those who have time for a little bit of a lesson, here you go:

Catholic and Protestant churches celebrate Easter on the first Sunday after the first full moon following the spring equinox.  The date is set according to the Gregorian calendar.  But of course it is not as simple as this.  The "full moon" is actually the paschal moon, which does not always correspond to the astronomical full moon. And the spring equinox is defined for simplicity's sake as March 21.  Paschal cycles, which have their origins in the sixth century, occur every 84 years.  Using this method Easter can occur any time between March 22 and the last Sunday in April.

Orthodox, Greek, and Russian churches use the same calculations but theirs are based on the Julian calendar and a paschal cycle of only nineteen years.

Sometimes the date of the Catholic / Protestant holiday coincides with that for the Orthodox / Greek / Russian "Pascha."  In 2013 Pascha takes place on May 5, 2013.

The Jewish festival of Passover takes place around the same time, and is centered around a seven or eight day period that begins on the fourteenth day of the Hebrew month of Nisan. Nisan is the first month of the ecclesiastical year (it is called "Aviv" in the Torah.)  This year Passover takes place March 25 through April 2.

Easter, Pascha, and Passover are regarded by many as the holiest days of the year.