The following is from William Kurz’s forthcoming commentary on Acts (November 2013), commenting on Acts 2:1–11:
The Jewish feast of Pentecost, or Weeks, fifty days after Passover, was one of the three great pilgrimage feasts of Israel (Deut 16:16). It was originally a harvest celebration but later also became a commemoration of God’s giving of the covenant and the law on Mount Sinai. For Christians, the coming of the Holy Spirit fulfills this meaning of Pentecost. The celebration of the gift of the law now embraces the giving of the new law in the Spirit (Rom 8:2), the writing of the law on the heart (Jer 31:31–34; 2 Cor 3:2–6).
The Spirit came on the disciples when they were all in one place together and united in prayer (1:14). The Spirit’s coming is manifested with observable signs: a noise like a strong driving wind and the appearance of tongues as of fire that rested on each of them. Throughout Scripture, wind is often a sign of the Spirit, since the same word (pneuma in Greek) means both wind and spirit. The loud noise, wind, and fire evoke the *theophanies on Mount Sinai to Moses (Exod 19:16–19) and Elijah (1 Kings 19:11–13). The sign of fire especially recalls the Baptist’s prophecy that the One to come “will baptize you with the holy Spirit and fire” (Luke 3:16). The sign that they were all filled with the holy Spirit which Luke most emphasizes, however, is that they began to speak in different tongues. In this context the Greek word glossai, literally “tongues,” is better translated as “other languages” (NRSV), since it was the ordinary word used for languages.