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The Festival of Pentecost occurs towards the end of the season of Easter, 50 days after Easter Sunday and 10 days after
Ascension. The name comes from 'Pentekoste', the Greek for 'fiftieth day'. Pentecost was originally the old Jewish festival of 'Shavuoth' or the 'Festival of Weeks', a form of Harvest Festival giving thanks for the first fruits of that year's crop, which the Jews celebrated 50 days
(7 weeks) after the festival of 'Passover'. Over the years, it had changed into a festival celebrating the giving of the 'Law' to Moses
(the Ten Commandments), which forms a major basis of the Jewish religion
(Exodus Chapters 19 & 20).
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The Apostles, and many other disciples including members of Jesus' own family, were gathered together to celebrate Shavuoth. Just before his ascension back to heaven, Jesus had told them to wait in Jerusalem for a 'comforter', a source of strength and guidance
(Luke 24:49). The festival of Pentecost celebrates that gift, the gift of God's Holy Spirit, which changed the old Jewish Festival of Shavuoth into the Christian Festival of Pentecost.
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It revolutionised their whole existence. From a demoralised group hiding in fear from the religious authorities, they were transformed into a confident group spreading the message of the Gospel throughout the world
('Gospel' = Greek for 'good news').