The resurrection of Jesus Christ from the dead is the center of the Orthodox Christian faith. Twelve weeks of preparation precede it. This is made up of pre-lenten Sundays, Great Lent, and Holy Week. The faithful try to make this long journey with repentance, forgiveness, reconciliation, prayer, fasting, almsgiving, and study. When the feast finally arrives, it is celebrated with a collection of services combined as one.
Pascha, also called Easter, is the feast of the Resurrection of the Lord. Pascha is a transliteration of the Greek word, which is itself a transliteration of the Hebrew pesach, both words meaning Passover. (A minority of English-speaking Orthodox prefer the English word 'Pasch.')
Pascha normally falls either one or five weeks later than the feast as observed by Christians who follow the Gregorian calendar. However, occasionally the two observances coincide, and on occasion they can be four weeks apart. The reason for the difference is that, though the two calendars use the same underlying formula to determine the festival, they compute from different starting points. The older Julian calendar's solar calendar is 13 days behind the Gregorian's and its lunar calendar is four to five days behind the Gregorian's.
Source: orthodoxwiki.org http://orthodoxwiki.org/Pascha
Source: A Reader's Guide
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