Pentecost, also called the Feast of Weeks, took place 50 days after Passover began. It is considered the second most important of all the Feasts of the Lord; it is one of the three occasions on which all males were required to appear before the Lord in the temple.
It was a reaffirmation of Yahweh's covenant with His people; the successful harvest (grain harvest) proved that He had taken care to sustain them through the previous year.
It was believed that Moses brought the Ten Commandments down from Sinai 50 days after the original Passover in Egypt, the feast became a commemoration of the giving of the Torah and the Covenant of Sinai.
For Christians, Pentecost celebrates the miracle of the birth of the Church, a dramatic affirmation that God's plan of redemption applies to all the world's people, not the Jews alone. The gift of the Holy Spirit that day was considered to be the dawn of a new age in which believers then and now were enjoined to become missionaries until the Gospel message is heard "to the end of the earth." The Holy Spirit empowers believers to take this message to all the earth.