What it is
Easter celebrates the resurrection of Jesus. Western churches set it as the first Sunday after the first full moon on or after March 21, using a church table rather than the astronomical moon. That puts it between March 22 and April 25.
Orthodox churches use the Julian calendar, so their Easter is often one to five weeks later.
Is it a day off?
Easter is always a Sunday, so it does not create a weekday holiday. Many stores close or keep short hours, and some schools time spring break around it.
History
The rule for Easter's date goes back to the Council of Nicaea in 325. This site calculates it with the standard Gregorian algorithm.