Easter, also called Pascha, is the most important religious feast in the Christian liturgical year. Christians celebrate this day in observance of their belief that Jesus rose from the dead on the third day after his crucifixion, now estimated to have taken place between the years AD 26 and AD 36. Many non-religious cultural elements have become part of the holiday, and those aspects are often celebrated by many Christians and non-Christians alike.
Easter also refers to the season of the church year called Eastertide or the Easter Season. Traditionally the Easter Season lasted for the forty days from Easter Day until Ascension Day but now officially lasts for the fifty days until Pentecost. The first week of the Easter Season is known as Easter Week or the Octave of Easter. Easter also marks the end of Lent, a season of prayer and penance.
Easter is termed a moveable feast because it is not fixed in relation to the civil calendar. Easter falls at some point between late March and late April each year (early April to early May in Eastern Christianity), following the cycle of the moon. After several centuries of disagreement, all churches accepted the computation of the Alexandrian Church (now the Coptic Church) that Easter is the first Sunday after the first fourteenth day of the moon (the Paschal Full Moon) that is on or after the ecclesiastical vernal equinox.
Easter is linked to the Jewish Passover not only for much of its symbolism but also for its position in the calendar. The Last Supper shared by Jesus and his disciples before his crucifixion is generally thought of as a Passover meal, based on the chronology in the Synoptic Gospels (Matthew 26:17; Mark 14:12; Luke 22:7). The Gospel of John, however, speaks of the Jewish elders not wanting to enter the hall of Pilate in order "that they might eat the Passover", implying that the Passover meal had not yet occurred (John 18:28; John 19:14). Thus, John places Christ's death at the time of the slaughter of the Passover lamb, which would put the Last Supper slightly before Passover, on 14 Nisan of the Bible's Hebrew calendar. According to The Catholic Encyclopedia, "In fact, the Jewish feast was taken over into the Christian Easter celebration."
The source of information: http://en.wikipedia.org/wiki/Easter