Pentecost - It always falls on the seventh Sunday after Easter. Pentecost means "fiftieth day": it is celebrated 50 days from Easter Sunday (including Easter Sunday in the counting), hence its name.
The Christian holiday of Pentecost is a moveable feast, which is celebrated on the 50th day from Easter Sunday. It commemorates the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ while they were in Jerusalem celebrating the Feast of Weeks, as described in the Acts of the Apostles. Wikipedia