What the text says. Pentecost is the feast of Weeks, fifty days after the offering of the first sheaf, and it is one of the three at which attendance was commanded (Deuteronomy 16:16). Acts opens its second chapter on that day and immediately explains the audience: "Now there were dwelling at Jerusalem Jews, devout men, from every nation under heaven" (Acts 2:5, ASV), followed by a list of some fifteen regions and peoples (Acts 2:9-11). Paul later organises a journey around the same date, sailing past Ephesus because "he was hastening, if it were possible for him, to be at Jerusalem the day of Pentecost" (Acts 20:16, ASV), and dates a stay by it in a letter (1 Corinthians 16:8).
Why the city was full of foreigners, the ordinary reason. The crowd in Acts 2 is not a miracle of assembly; it is a pilgrimage. By the first century large Jewish communities lived permanently across the Roman and Parthian worlds, Alexandria, Rome, Asia Minor, Mesopotamia, Cyrene, and the pilgrimage obligation drew a share of them to Jerusalem for the appointed feasts. Josephus confirms independently that Pentecost brought crowds in: describing the disturbances after Herod's death, he writes that when "that feast, which was observed after seven weeks, and which the Jews called Pentecost, [i. e. the 50th day,] was at hand" the people gathered, and "an immense multitude ran together, out of Galilee, and Idumea, and Jericho, and Perea, that was beyond Jordan" (Wars 2.3.1, Whiston's translation). ⚠️
Note what that passage is and is not: on that particular occasion Josephus says plainly that the people gathered "not on account of the accustomed Divine worship, but of the indignation they had", it was a political gathering at a festival date. It is good evidence that the feast drew large numbers to the city; it is not a description of ordinary festival piety.
Sources: Acts 2:1, 2:5, 2:9-11; Acts 20:16; 1 Corinthians 16:8; Leviticus 23:15-16; Deuteronomy 16:9-11, 16:16; Josephus, Wars 2.3.1; ISBE 1915, "Pentecost" and "Dispersion".
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