Pentecost was originally an Old Testament festival, since the time of Josephus calculated as beginning on the fiftieth day after the beginning of Passover. In the Christian calendar, it falls on the seventh Sunday after Easter. It was called the Feast of Weeks (Shavuot), and in the Old Testament was originally an agricultural festival celebrating and giving thanks for the "first fruits" of the early spring harvest (Lev 23, Exod 23, 34).
By the early New Testament period, it had gradually lost its association with agriculture and became associated with the celebration of God’s creation of His people and their religious history. By the destruction of Jerusalem in AD 70, the festival focused exclusively on God’s gracious gift of Torah (the "Law") on Mount Sinai. It continues to be celebrated in this manner in modern Judaism.
While there are other references to Pentecost in the New Testament (for example, 1 Cor 16:8), it is most significant in Acts 2 and the familiar scene of the outpouring of the Holy Spirit on those in the "upper room." The New Testament writers associate the events of Acts 2 with Pentecost, and relate it to the prophecies of Joel 2 and promises of Jesus (Acts 1:8). In both, the emphasis is on a empowerment through the Holy Spirit to enable the people of God to witness to Jesus the Christ.
Pentecost represents God’s gracious, enabling presence actively at work among His people, calling and enabling them to live out in dynamic ways the witness of being His people. Perhaps at this point there is direct contact with the Pentecost of Judaism, for in Judaism the Torah, God’s instruction to His people, is the means by which they become His witness to the world.
The word “pentecost” means “fiftieth” and refers to the fiftieth day. In most Christian traditions, Pentecost Sunday occurs 50 days following Easter Sunday (counting Easter Sunday since it is the first day of the week). Those 50 days span seven Sundays after Easter, so Pentecost is the seventh Sunday after Easter (7 weeks times 7 days = 49 days, plus Pentecost Sunday). Since Easter is a “movable feast,” meaning that it occurs on different days in different years (it is tied to the lunar cycle while the calendar is solar based), Pentecost is also moveable. It can occur as early as May 10 and as late as June 13 (see The Church Year for current dates). Some Christian traditions, Eastern Orthodox for example, use a different religious calendar and so have different dates for much of the Christian Year. Also, in Eastern church traditions, Pentecost can also refer to the entire fifty days between Easter and Pentecost.
For Christians, Pentecost Sunday is a day to celebrate hope, a hope evoked by the knowledge that God through His Holy Spirit is at work among His people. It is a celebration of newness, of recreation, of renewal of purpose, mission, and calling as God’s people. It is a celebration of God’s ongoing work in the world. Yet, it is also a recognition that His work is done through His people as He pours out His presence upon them.