The Feast of Pentecost: Held Fiftieth Day After offering First Sheaf of Barley
Source: TTT
Leviticus 23:15, 16
From the day after the Sabbath, the day you brought the sheaf of the wave offering, you are to count off seven full weeks. / You shall count off fifty days until the day after the seventh Sabbath, and then present an offering of new grain to the LORD.
Deuteronomy 16:9
You are to count off seven weeks from the time you first put the sickle to the standing grain.