The church is so used to associating the outpouring of the Holy Spirit with Pentecost that we often forget that Pentecost was a holiday before this moment. With this in mind, we shouldn’t think that God sent the Holy Spirit on a day that just-so-happened to be Pentecost, but rather that He intentionally chose this day to further reveal something to His people. If Jesus decided to come back and save us on a particular American holiday, we would take note of the symbolism right? And so, in the same way, we should expect that the Jews saw some meaning in the fact that the Holy Spirit came on this particular holiday.
So then why did God choose Pentecost? What was the connection? Well, that connection isn’t clear at first glance, because the meaning of Pentecost had evolved by Jesus’ time. By Old Testament accounts, Pentecost was connected to the Passover. At the beginning of Passover you were to make an offering of a barley sheaf and 50 days after that you were to celebrate Pentecost, which literally means “fiftieth.” Pentecost was also known as “the feast of weeks” (Ex 34:22; Dt 16:10), “the feast of the harvest,” (Ex 23:16) and “the day of the firstfruits” (Nu 28:26).
This celebration marked the completion of the barley harvest and on this day the Israelites were to bring two loaves of bread as an offering to God as the firstfruits (Lev 23:17-20). This was Pentecost’s original significance. However, “By the middle of the second century bc, Pentecost had become a covenant renewal celebration (Jubilees 14.1–6; 15.1–16; 22.1–9),” (Roberts, Ronald D. “Pentecost.” The Lexham Bible Dictionary) and was a time “for making and remembering the biblical covenants and for renewing the Sinaitic covenant.” (VanderKam, James C. “Covenant and Pentecost.” Calvin Theological Journal, vol. 37, 2002, p. 253.)
Theologian Michael J. Gorman also sees the theme of covenant active in the Pentecost celebration of Jesus’ time, stating that, “Luke’s dramatic narrative of Pentecost itself is his way of stating (among other things) that the Spirit-filled community of the new covenant promised by Jeremiah and Ezekiel has arrived.” (Gorman, Michael J. The Death of the Messiah and the Birth of the New Covenant. Eugene, Cascade Books, 2014, p. 40.) Gorman goes on to call Pentecost, “a lovely merger of Sinai, temple, and new-covenant themes—and much more.” (Ibid. p. 50.)
With this in mind, Pentecost partially becomes the story of how Moses’ old Sinaitic covenant was replaced by Jesus’ superior new covenant. In the old covenant, God passes down the law, but in the new covenant God passes down the Spirit.
We can further see the Holy Spirit as a symbol of the new covenant poured out at Pentecost when we read Paul’s words in Ephesians 4:8. It’s there that Paul applies Psalm 68:18 to Jesus and says, “when he ascended on high he led a host of captives, and he gave gifts to men.” This statement may not seem controversial, but once you look at the rabbinic tradition of the time, you begin to see that this Psalm was often associated with Moses and not the Messiah. As one commentary explains,
The application of Psalm 68:18 to Christ’s ascent and subsequent distribution of gifts stands in contrast to the rabbinic tradition as reflected in the Targum which associates Psalm 68:18 with Moses’ ascent of Mount Sinai, an ascent that was understood as a going up to heaven to receive the Torah and other heavenly secrets. The ‘Moses mysticism’ associated with this interpretation of the Psalm was widespread and apparently early, appearing in the rabbinic writings and Philo. If this background is in view, Paul may be deliberately presenting Christ as greater than Moses. He ascended far above all the heavens in order to fill all things ([Eph] v. 10). His gift is not the Torah but grace ([Eph] v. 7), while his special gifts of ministry are for building up the whole body ([Eph] vv. 11–16), not heavenly secrets for an elite few. The liturgical custom in synagogues associated Psalm 68 with Pentecost, which was increasingly regarded by Jews as the feast which commemorated the giving of the law to Moses on Mount Sinai. This has suggested to some Christian scholars that Pentecost lies in the background to the apostle’s handling of the Psalm here. (O’Brien, Peter Thomas. The Letter to the Ephesians. Grand Rapids, William B. Eerdmans Publishing Company, 1999. p. 291. The Pillar New Testament Commentary.)
But outside of the symbolic meaning of Pentecost, there’s a practical reason God chose this holiday to endow His people with the Holy Spirit; for on this day “there were dwelling in Jerusalem Jews, devout men from every nation under heaven” (Ac 2:5). It becomes clear that part of the reason God gave the Christians gathered in Jerusalem the gift of tongues, was so that they could reach all the nations that gathered in Jerusalem for the holiday.