Shavu'ot (Pentecost, the Feast of Weeks, the Feast of Harvest) is 50 days from the first Sunday that follows The Passover. It is the second of God's three ordained Annual High Sabbaths ("Pilgrimage" Sabbaths).
In CE 2008, the true Biblical Pentecost (Shavu'ot) fell on a different date than the traditional Christian "Pentecost Sunday" — demonstrating once again that the false Pentecost counted from the capricious Easter does not align with the Biblical calendar.
In the year of Christ Jesus' ascension, the Holy Spirit descended on the disciples on the true Shavu'ot — Pentecost — exactly as God had ordained. This event, recorded in Acts 2, is the birthday of the Christian Church.
See also FAQ 06 — What Are the Hebrew High Holy Days? and Easter, Passover & Pentecost Chart for exact dates.