The feast of Pentecost was in NT times the name for the celebration of the Feast of Weeks because it occurred on the fiftieth day after Passover. It was a one-day festival in which special sacrifices were offered, and originally it was a harvest (firstfruits) festival (Exod. 23:16;34:22; Lev. 23:15–21; Num. 28:26; Deut. 16:9–12). It is possible, but not certain, that as early as this time this festival was associated with the giving of the Law on Sinai.
Witherington, Ben, III. The Acts of the Apostles: A Socio-Rhetorical Commentary. Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1998. Print.