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Whitsun, or Pentecost as it is also known, is a Christian feast celebrated on the 49th day (7 weeks) after Easter Sunday. Pentecost falls on the tenth day after Ascension Thursday. Historically and symbolically related to the Jewish harvest festival of Shavuot, or the day on which God gave the Ten Commandments at Mount Sinai. Pentecost also commemorates the descent of the holy spirit upon the Apostles and other followers of Jesus as described in the Book of Acts, Chapter 2 in the New Testament. For this reason, Pentecost is sometimes described as "the church's birthday."
In the story recounted in the Acts of the Apostles, the apostles went out into Jerusalem prophesying and speaking in languages that all the visitors to Jerusalem could understand ("God-fearing Jews from every nation under heaven...each one heard them speaking in his own language"). This is where the name of the Pentecostalist Church denomination comes from, since Pentecostalist worship often includes the practice of speaking in tongues.
According to legend, King Arthur always gathered all his knights at the round table for a feast and a quest on Pentecost. So ever the King had a custom that at the feast of Pentecost in especial, afore other feasts in the year, he would not go that day to meat until he had heard or seen of a great marvel.