Feast of Firstfruits
Waving of the Sheaf Offering
(Lev 23:9) And the LORD spake unto Moses, saying,
(Lev 23:10) Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest:
(Lev 23:11) And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it.
(Lev 23:10) Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest:
(Lev 23:11) And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it.
(Num 28:26) Also in the day of the firstfruits, when ye bring a new meat offering unto the LORD, after your weeks be out, ye shall have an holy convocation; ye shall do no servile work:
The Feast day of the waving of the offer could not occur on the Sabbath DURING the Feast of Unleavened Bread but the morrow after the following Sabbath.
From this day you count 7 weeks and, on the day after, you get to Shavuot.
The Feast of Weeks therefore had a beginning date (Waving of the Sheaf offering - Day 26 of the first month) and an ending date (Shavuot - Day 15 of the third month) and both had to be kept as Sabbaths.