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Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday (also Resurrection Day or Resurrection Sunday). The chronology of his death and resurrection is variously interpreted to be between AD 26 and 36.
Easter marks the end of Lent, a forty-day period of fasting, prayer, and penance. The last week of the Lent is called Holy Week, and it contains Good Friday, commemorating the crucifixion and death of Jesus. Easter is followed by a fifty-day period called Eastertide or the Easter Season, ending with Pentecost Sunday.
Easter is a moveable feast, meaning it is not fixed in relation to the civil calendar. The First Council of Nicaea (325) established the date of Easter as the first Sunday after the full moon (the Paschal Full Moon) following the northern hemisphere's vernal equinox. Ecclesiastically, the equinox is reckoned to be on March 21 (even though the equinox occurs, astronomically speaking, on March 20 in most years), and the 'Full Moon' is not necessarily the astronomically correct date. The date of Easter therefore varies between March 22 and April 25.
Eastern Christianity bases its calculations on the Julian Calendar whose March 21 corresponds, during the 21st century, to the 3rd of April in the Gregorian Calendar, in which calendar their celebration of Easter therefore varies between April 4 and May 8.