Jamieson, Fausset and Brown on Deuteronomy 16:9-12

9-12. Seven weeks shalt thou number--The feast of weeks, or a WEEK OF WEEKS: the feast of pentecost (see on Le 23:10; also see Ex 34:22; Ac 2:1). As on the second day of the passover a sheaf of new barley, reaped on purpose, was offered, so on the second day of pentecost a sheaf of new wheat was presented as first-fruits (Ex 23:16; Nu 28:26), a freewill, spontaneous tribute of gratitude to God for His temporal bounties. This feast was instituted in memory of the giving of the law, that spiritual food by which man's soul is nourished (De 8:3).