When did Pentecost happen?

Answer 1

It happened before the Christian calender was created, when the Roman calender was still used - and no one took notes, so no one really knows.

Answer 2

Pentecost is the Greek name for the Jewish pilgrimage festival of Shavuot (also called the Feast of Weeks). This occurs 50 days after the first evening of Passover, or 7 times 7 weeks (plus one because the count of one is on the second night of Passover). To this day, Jews celebrate Shavuot; it is both a harvest festival (the winter wheat harvest) and a festival celebrating the anniversary of God's revelation at Mount Sinai. Christians celebrate Pentecost because the disciples were gathered for the Jewish festival when they experienced what Christians call the gifts of the Holy Spirit. The Christian and Jewish calendars have diverged since then, so the two communities do not celebrate it at the same time.