The Shavuot Musaf-offering
Requires Beit HaMikdash
BaMidbar 28:26-27 On the day of the first fruits when you bring a new mincha-offering to Hashem on your Festival of Shavuot, there shall be a sacred holiday to you when you shall not do any work of consequence.
We are commanded to bring a Musaf (Additional)-offering also on the 50th day after the offering of the Omer, which is the 16th day of Nisan. This is the Musaf-offering of the Festival of Shavuot, mentioned in the book of BaMidbar.
It is to be noted that Scripture does not specify the exact day of the month in which the Festival of Shavuot falls, but merely states that it is to be celebrated seven full weeks after the offering of the Omer. The seven weeks were counted from the 16th of Nisan, so that in ancient Yisrael, when the New Moon was determined by actual observation (P153), the Festival of Shavuot sometimes fell on the 5th, sometimes on the 6th, and sometimes on the 7th of Sivan. After the introduction of the fixed calendar, which determined that Nisan and Iyar (following it) should invariably contain 30 and 29 days respectively, the Festival of Shavuot came to fall invariably on the 6th of Sivan. In the Galut (Diaspora-outside Yisrael), the Festival is observed on the 6th and the 7th of Sivan.