Pentecost is a holiday of Christianity. Pentacost is seven weeks after Easter (or 50 days after Pascha). It is also just 10 days after the Ascension. It comes from the Greek word Pentékosté, meaning "fiftieth", and originally referring to Shavuot (the second day of Passover, on the 16th of Nisan, is the first day of counting the Omer; Shavuot is celebrated after seven full weeks, on the fiftieth day).
Christian churches that emphasize the Holy Spirit celebrate Pentecost, also called Whitsun, as the anniversary of the disciples' being filled with the Spirit, as described in the New Testament in Acts chapter 2. Most Christians recognize this event as the birth of the Church.