I have written a longer article about the OT roots of the celebration of Pentecost that you can read here. This is a summary of that post, intended for use as a Bible Study.
Of the 364 other days of the year upon which Christ could have poured out His Holy Spirit, why did He do so on exactly the fiftieth day after Easter? What was so important about this day? It was, indeed, already a holy day, the OT Feast of Weeks. But why choose this feast day? What makes Weeks so fitting a time for Jesus to give His Holy Spirit to the church?
The OT Feast of Weeks
The second major festival of the Israelite liturgical calendar was Pentecost or the Feast of Weeks (Shavuot in Hebrew). The “feast of Weeks” is more exactly the feast of seven weeks, for beginning on the day after Passover (the 16th of Nisan), the Israelites counted forty-nine days, then commenced the celebration of the feast of Weeks on the following day (Lev 23:15-16; Deut 16:9-10). Because it fell on the fiftieth day after Passover, Weeks was also called “Pentecost”, that is, “fiftieth” (e.g., Acts 2:1; 20:16; 1 Cor 16:8).
Pentecost is an agricultural festival. Believers presented to the Lord two loaves of bread, made from fine flour, and baked with leaven, as the first-fruits of the wheat harvest. In addition to the grain offering, they offered one bull, two rams, seven lambs, along with a sin offering of a male goat, and two male lambs for a peace offering (Lev 23:15-19; Num 28:26-31). Since the first sheaf of the barley harvest was presented to YHWH on the day after Passover (Lev 23:11), and the first sheaf of the wheat harvest was offered fifty days later (23:15), Passover and Pentecost marked the beginning and end of the grain harvest.
Pentecost and the Giving of the Law at Sinai
Over time, Pentecost came to be celebrated as the anniversary feast of the giving of the Law or the establishment of the covenant at Sinai. This is attested in Jewish writings such as Jubilees (1st century BC) and the Babylonian Talmud. There are hints in the OT itself, however, that Pentecost was linked to the giving of the law. In Exod 19:1, Moses writes that the Israelites arrived in the wilderness of Sinai “in the third month” after they had left Egypt. Since they left on the day after Passover, in the middle of the first month (Exod 12:2, 6), the fiftieth day after Passover would have fallen within this third month. Also, in 2 Chr 15:10-15, the Chronicler describes a gathering in Jerusalem, during the third month, where the covenant was celebrated and renewed. A later Aramaic paraphrase of Chronicles, called a Targum, says expressly that the Israelites gathered in Jerusalem during the festival of Weeks.
Pentecost and the Jubilee Year
The Israelites celebrated the Jubilee Year during the fiftieth year following every “seven sabbaths of years” or forty-nine years (Lev 25:8-55; 27:16-25; Num 36:4). During this year, any ancestral land that Israelites families had sold was given back to them. Also, any Israelite who, induced by poverty, had sold himself (or been sold) into slavery to a fellow Israelite regained his liberty. Not only the people, but the land itself was “freed” from being worked. No planting or sowing, harvesting or reaping took place during the fiftieth year. Like the sabbatical year (every seventh year), the jubilee year was a great sabbath or rest for the people of YHWH and the land that belonged to him. Therefore, because of the Jubilee Year, the number fifty is closely associated with the remission of debts, emancipation of slaves, and rest within God’s protective care. Like the festival held every fifty years, so the festival held every year on the fiftieth day proclaimed the following: (1) God had freed the Israelites from slavery in Egypt; (2) he had fulfilled his promise to give them the Holy Land; (3) he provided rest for them from their labors.
The Christian Pentecost as the Fulfillment of the OT Feast of Weeks
1. Fifty Day Period of Anticipation: As the days between Passover and Pentecost were symbolic of the days of waiting between the Israelites’ departure from Egypt and entrance into Canaan, when they could finally offer the first-fruits from the soil of the holy land, so these days between the Passover resurrection of Jesus and the giving of the first-fruits of the Spirit on Pentecost were days of waiting (Luke 24:49; Acts 1:4).
2. First-Fruits of the Spirit: As the Israelites celebrated Pentecost, they offered to God the first-fruits of the wheat harvest (Lev 23:17), but at the new Pentecost, God offered to His church the first-fruits of the Spirit (Rom 8:23; Eph 1:13-14). By offering to God the first-fruits of grain, the believer bore witness that whole field and crop belonged to God, whose continued blessing was requested through the sacrifice itself. Similarly, Christ places the Spirit within the believer as a pledge that the whole person, body and soul, belongs to him. He will continue to care for that person in whom the first-fruits of the Spirit are present until the “full harvest,” as it were, of the resurrection of the flesh.
3. Divine Speech from Divine Fire: When the law was given from Sinai, God appeared in a “thick cloud” (Exod 19:9); at the sound of a “ram’s horn” (19:13); with “thunder and lightning flashes” (19:16); and in “smoke...like the smoke of a furnace,” (19:18). And “the mountain was burning with fire unto the heart of the heavens: darkness, cloud, and thick darkness,” (Deut 4:11). Then the Lord spoke to the Israelites “from the midst of the fire," (4:12, 15, 33; cf. 5:22-26). He “showed [them] his great fire and [they] heard his words from the midst of the fire,” (4:36). At Jerusalem, on the other hand, there was the “rushing of a violent wind” from heaven (Acts 2:2); “divided tongues, as of fire, which rested upon each one of them,” (2:3); and the apostolic proclamation(s) of the Gospel in unlearned languages. In both cases, there was divine speech connected with divine fire, but the message could not have been more different.
4. The Preaching of a New Covenant: If the OT Pentecost was an annual celebration of the giving of the first covenant, then the NT Pentecost is an annual celebration of the giving of the new covenant. This new covenant was prophesied by Jeremiah (31:31-34); established by Jesus at the Last Supper (Luke 22:20); and preached by the apostles at the pouring out of the Spirit (Acts 2). Christ laid upon the listeners not the “ten words” for them to fulfill; rather, he proclaimed the fulfillment of the law of Moses, the prophets, and the psalms in himself (cf. Luke 24:44).
5. The Year of Jubilee: During the OT year of Jubilee, there was a focus on freedom from bondage, the gift of the holy land, and rest from labor. Jesus bestowed all three of these gifts in greater measure during his ministry (Luke 4:18-19). The Spirit who anointed Jesus to work these deeds is the same Spirit who came upon the apostles at Pentecost to preach freedom from sin, the gift of the kingdom of God, and rest in the atoning work of Jesus Christ.