What is the celebration of Pentecost?

The Feast of Pentecost (or Weeks) was one of the major Jewish feasts. Acts 2:5-11 describes the large number of Jewish pilgrims who had traveled to Jerusalem for its observance.

"Pentecost" literally means "the fiftieth day," and it was celebrated fifty days following the Passover Sabbath. Just as the Passover was to commemorate the Exodus from Egypt, Pentecost was to commemorate the giving of the Law on Mt. Sinai. [See also Exodus 23:15, 16; Ex. 34:22; Leviticus 23:16; Numbers 28:26; Deuteronomy 16:10.]

It was on the occasion of this feast that the Holy Spirit was poured out on Jesus' apostles (Acts 2:1-4) and believers were first said to be "added" to Christ's church (Acts 2:41-47), thus its significance to Christianity as the "Day of Pentecost" on which the church was established.