John Gill Commentary Deuteronomy 16:9

"Seven weeks shalt thou number unto thee: from the time thou beginnest to put the sickle to the standing grain shalt thou begin to number seven weeks." — Deuteronomy 16:9 (ASV)

Seven weeks then you shall number for yourself
And then another feast was to take place, called from hence the feast of weeks, and sometimes Pentecost, from its being the fiftieth day:

begin to number the seven weeks from such time as you begin to put
the sickle to the corn ;
for the sheaf of the wave offering, as the first fruits of barley harvest, which was done on the morrow after the sabbath in the passover week, and from there seven weeks or fifty days were reckoned, and the fiftieth day was the feast here ordered to be kept; so the Targum of Jonathan, "after the reaping of the sheaf you shall begin to number seven weeks;" see (Leviticus 23:15) .