Pentecost is the celebration commemorating the coming of the Holy Spirit and the birth of the Church fifty days after the resurrection of Jesus (hence its name) and ten days after His ascension.
It is also called Whit Sunday, Whitsun, or Whitsuntide. These names are from an Anglo-Saxon contraction of "White Sunday.” White (or whit) was not only a symbol of purity throughout Christendom, but also of “wit” or understanding—thus, the association with the outpouring of the Spirit upon the redeemed.
Pentecost corresponds to the Jewish festival of Shavuot, which commemorates God giving the 10 Commandments at Mt. Sinai fifty days after the Exodus.
So it is that on this day, we who are named with the Name of Christ, rejoice at the giving of His Spirit and all the good gifts therein.