Easter is the most important festival in the Christian calendar, alongside Christmas, and marks the day when, as Christians believe, Jesus of Nazareth rose from the dead after having been crucified on the previous Friday (“Good Friday”). The exact year of his crucifixion is unknown, although the year usually cited is 33 CE.
It’s a moveable feast (ie, its date is calculated each year on the basis of the moon’s phases), and as a result it’s often celebrated on different days by different Christian sects. “Western” Christians (Roman Catholics and Protestants, and some Orthodox) and some “Eastern” (Orthodox, Coptic, Syrian etc) Christians celebrate it according to the Gregorian calendar in general use worldwide. Some Eastern Christians calculate religious festivals by the Julian calendar, which currently runs 13 days behind the Gregorian calendar. In 2015, the Western churches celebrate it on 5 April; the Eastern churches celebrate a week later, on 12 April. In 2014, Easter Sunday happened to fall on the same day for both: 20 April.
It’s closely related to the Jewish festival of Passover and in most European languages its name is derived from that festival’s Hebrew name (Pesach). The origin of the English name of Easter (and German Ostern) is unclear, but appears to derive from a pre-Christian goddess of the dawn, Ēostre. Either way, it seems likely that the Christian festival was superimposed on an existing Pagan one.
These days, of course, Easter’s associated as much with chocolates (particularly in egg form) as with Jesus. Which is odd, because there’s no mention of chocolate in the Bible. But it’s a good excuse for a sweet treat!