Negative commandment · Lo Ta’aseh #325 of 613
Not to work on the holiday of Shavuot
שלא לעשות מלאכה בחג השבועות
The verse · Vayikra 23:21
וּקְרָאתֶם בְּעֶצֶם הַיּוֹם הַזֶּה מִקְרָא־קֹדֶשׁ יִהְיֶה לָכֶם כׇּל־מְלֶאכֶת עֲבֹדָה לֹא תַעֲשׂוּ חֻקַּת עוֹלָם בְּכׇל־מוֹשְׁבֹתֵיכֶם לְדֹרֹתֵיכֶם
“And you shall proclaim on this very same day, that it may be a holy gathering to you: you shall do no servile work: it shall be a statute for ever in all your dwellings throughout your generations.”
The Rambam’s words
Not to work on the holiday of Shavuot, as [Leviticus 23:21] states concerning it: "Do not do any servile work."
שלא לעשות מלאכה בחג השבועות, שנאמר בו "כל מלאכת עבודה, לא תעשו" (ויקרא כג,כא; במדבר כח,כו).
Why · Sefer HaChinuch #309
To not do work on the day of the holiday of Shavuot
“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.”
Where in the Mishneh Torah
Hilchot Rest on a Holiday הלכות שביתת יום טוב