Origin of the Feast of Weeks
The Feast of Weeks takes place on the 50th day after the Day of Firstfruits.
Forty days after crossing the Red Sea, Moses went up to Mount Sinai for the first time to receive instructions from God and deliver God’s will to the people. Ten days later (50 days after crossing the Red Sea), Moses returned to Mount Sinai and received the Ten Commandments from God (Ex 19:16-25).
God commanded the Israelites to commemorate the day when Moses went up Mount Sinai to receive the Ten Commandments as the Feast of Weeks. There were seven Sabbaths from the Day of Firstfruits when the Israelites came out of the Red Sea until the day Moses went up to Mount Sinai to receive the Ten Commandments.
“From the day after the Sabbath, the day you brought the sheaf of the wave offering, count off seven full weeks. Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the Lord.”
Leviticus 23:15-16 (NIV)