Hebrew Shavuot, "weeks" (from counting seven weeks); Greek Pentēkostē, "fiftieth" (from counting fifty days from Passover). The second of Israel's three pilgrim festivals (Exodus 23:16, Leviticus 23:15-22, Deuteronomy 16:9-12), celebrated on the 50th day after the Sabbath of Passover (hence "Pentecost"). Biblically it commemorates the wheat harvest (Exodus 34:22) with the offering of two loaves of leavened bread and various other sacrifices. Later Jewish tradition associated it with the giving of the Law at Sinai (calculated as 50 days after the Exodus) — a tradition that enriches but does not displace the biblical agricultural focus.
Shavuot's significance in the Christian calendar is massive because of what happened on one particular Shavuot around AD 30. The Holy Spirit fell at Pentecost (Acts 2) — the 50th day after Jesus' resurrection Passover. The parallel is striking: (1) First Shavuot (traditional): Law given to Israel from Mount Sinai — 3,000 Israelites died (Exodus 32:28) after the golden calf. Pentecost of Acts 2: Spirit given to the Church — 3,000 were saved (Acts 2:41). The Law kills apart from the Spirit; the Spirit gives life apart from the Law's condemnation. (2) Two loaves leavened at Shavuot (Leviticus 23:17) — unusual because most offerings required unleavened bread; the leavened loaves arguably pictured Jew and Gentile together in the Church, imperfect but accepted. (3) Firstfruits — Shavuot offered the firstfruits of the wheat harvest; Pentecost inaugurated the great harvest of nations through the Spirit's empowered Church. The first gospel sermon under the Spirit yielded 3,000 converts from 15 nations listed in Acts 2:9-11, a rehearsal of the final harvest of Revelation 7:9. (4) Completion of Passover. The Exodus was incomplete without Sinai; the resurrection was incomplete without Pentecost. Both feasts are now fulfilled in Christ and His Spirit.