Acts 2:1

2:1 day of Pentecost. Lit. the “Fiftieth Day” after the Sabbath of the Passover week (Lev. 23:4–7, 15–16). Pentecost was celebrated on the first day of the week and was one of the three great annual feasts of Israel, preceded by Passover (Lev. 23:4–8; Num. 28:16–25) and followed four months later by the Feast of Booths (Lev. 23:33–43; Num. 29:12–38; cf. John 7:1–44). Pentecost is also called the “Feast of Weeks,” because it was celebrated seven weeks after Passover (Deut. 16:10); the “Feast of Harvest,” because the first fruits of the harvest were gathered then (Ex. 23:16); and the “day of the firstfruits” (Num. 28:26).

they were all together. All of the apostles (1:16) were there, and probably many of the 120 mentioned earlier (1:15).