Sefer HaChinuch ספר החינוך

To not do work on the day of the holiday of Shavuot: To not do work on the day of [Shavuot] — which is the sixth day of Sivan, as it is stated (Leviticus 23:15-16), “And you shall count for yourselves from the morrow of the Shabbat, etc. you shall count fifty days, etc.” And the understanding of “from the morrow of the Shabbat,” is meaning to say, the morrow of the first holiday of Pesach, about which it was speaking first. As if it was the Shabbat of creation (the seventh day of the week) — if so, it will not have informed us which. And it comes out that the fifty days end on the sixth of Sivan. How is this? Fifteen days from Nissan, which is always full (consisting of thirty days); and twenty-nine days of Iyar, which is always lacking (consisting of twenty-nine days); and six days of Sivan — behold, [that is] fifty. And this fiftieth day — which was the day that the Torah was given — is the holiday of the Assembly (atseret), and it is also called the holiday of Shavuot (Weeks). And it is also written at the end of the section about this glorious day (Leviticus 23:21), “all work of labor shall you not do” — we have already written that any labor that is not for the needs of food for the soul is called, work of labor.

The root of the commandment of this appointed time is hinted in the commandment of the counting of the omer (Sefer HaChinukh 306) — and take it from there.

And I have written some of its laws — as is my custom — in the commandment of the resting from work on the first day of Pesach in this Order (Sefer HaChinukh 298). And there is no need to write at length about the rest of the appointed times of the year. As the six well-known days of the appointed times of the year — and they are the first and seventh [days] of Pesach, the first and eighth days of the Holiday [Sukkot], Atseret [Shavuot] and Rosh Hashanah — all have one law for all of their dispensations and all of their prohibitions. And the elucidation of all of its laws at length is in the tractate that is built on this, and that is Tractate Beitzah. (See Tur, Orach Chaim 496.)