Acts 2:1

The day of Pentecost was an annual spring feast at which the Jews presented the first-fruits of their wheat harvest to God (Exodus 34:22 a). The Jews also called Pentecost the Feast of Harvest and the Feast of Weeks in earlier times. They celebrated it at the end of seven weeks (i.e., a week of weeks) following the Feast of Passover. God received a new crop of believers, Christians, on this particular day of Pentecost. The Jews also celebrated Pentecost as the anniversary of the giving of the Mosaic Law (cf. Exodus 19:1). Paul regarded the Spirit’s indwelling presence as God’s replacement for the external guidance that the Mosaic Law had provided believers under that old covenant (Galatians 3:3; Galatians 3:23-29).

"Pentecost" is a Greek word, transliterated into English, that means fiftieth. This feast fell on the fiftieth day after Passover. It was one of the feasts at which all the male Jews had to be present at the central sanctuary (Exodus 34:22-23). Jews who lived up to 20 miles from Jerusalem were expected to travel to Jerusalem to attend these feasts. Pentecost usually fell in late May or early June. Travelling conditions then made it possible for Jews who lived farther away to visit Jerusalem too. These factors account for the large number of Jews present in Jerusalem on this particular day.

". . . by paralleling Jesus’ baptism with the experience of Jesus’ early followers at Pentecost, Luke is showing that the mission of the Christian church, as was the ministry of Jesus, is dependent upon the coming of the Holy Spirit. And by his stress on Pentecost as the day when the miracle took place, he is also suggesting (1) that the Spirit’s coming is in continuity with God’s purposes in giving the law and yet (2) that the Spirit’s coming signals the essential difference between the Jewish faith and commitment to Jesus, for whereas the former is Torah centered and Torah directed, the latter is Christ centered and Spirit directed-all of which sounds very much like Paul." [Note: Longenecker, p. 269.]

The antecedent of "they" is apparently the believers Luke mentioned in Acts 1:15. It is not possible to identify the place (lit. the house, Gr. ton oikon) where they assembled certainly. Perhaps it was the upper room already mentioned (Acts 1:13) or another house. Clearly the disciples were indoors (Acts 2:2).