Easter, or more properly "Resurrection Sunday," is the Christian holiday marking the resurrection of Jesus Christ three days after His crucifixion on Good Friday, and is the most important festival of the liturgical year. Its name is derived from Eastre, the pagan goddess of fertility and spring. In biblical times the Jewish Passover, which commemorates the sparing of the Hebrew firstborns, was celebrated at roughly the same time as the spring equinox or this “Eastre” festival. As the Gospel was being spread to the pagans, the celebration of Eastre and the celebration of the resurrection became combined and the Christians adopted the name that the pagans had used for their festival. In 325, the Nicaean Counsel declared that the Easter holiday would be celebrated on the first Sunday after the full moon of the vernal equinox.
Easter doesn't fall on a fixed date in the calendar. Instead, it is based on a lunar calendar like that used by the Jews. At the Nicaean Council in 325 AD it was decided that Easter would be celebrated on the Sunday after the first full moon on or after the day of the vernal equinox. The date of the full moon was to be determined by using the Metonic Cycle. Eastern Orthodox Churches still use the Julian calendar when determining the date of Easter and thus have an inaccurate date for the resurrection.
Easter marks the end of the 46 days of Lent, a period of fasting and penitence in preparation for Easter, which begins on Ash Wednesday and ends on Easter Sunday. However, Lent really only comprises 40 days since the 6 Sundays during this period are excluded from the Lent fast, and are days that are set apart to commemorate Easter Sunday and Jesus Christ, who rose from the dead, defeated the grave and guaranteed eternal life to those who believe.
The days before Easter also are special in the Christian tradition: the Sunday before is Palm Sunday, and the last three days before Easter are Holy Thursday, Good Friday and Silent Saturday. These days commemorate Jesus' entry into Jerusalem, the Last Supper and the Crucifixion.
Pentecost is seven weeks after Easter.