Shavuot שבועות

Leviticus 23:15-16

וּסְפַרְתֶּם לָכֶם מִמָּחֳרַת הַשַּׁבָּת מִיּוֹם הֲבִיאֲכֶם אֶת־עֹמֶר הַתְּנוּפָה שֶׁבַע שַׁבָּתוֹת תְּמִימֹת תִּהְיֶינָה׃

^23:15 And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;

עַד מִמָּחֳרַת הַשַּׁבָּת הַשְּׁבִיעִת תִּסְפְּרוּ חֲמִשִּׁים יוֹם וְהִקְרַבְתֶּם מִנְחָה חֲדָשָׁה לַיהוָה׃

^23:16 even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the LORD.

JPS 1917 and Tanach with Nikkud, public domain, via Sefaria

Between Passover and Shavuot there's a 49-day count, called the Omer. This is where it comes from. The festival at the end is named for the weeks you just counted.

Exodus 19:16-19

וַיְהִי בַיּוֹם הַשְּׁלִישִׁי בִּהְיֹת הַבֹּקֶר וַיְהִי קֹלֹת וּבְרָקִים וְעָנָן כָּבֵד עַל־הָהָר וְקֹל שֹׁפָר חָזָק מְאֹד וַיֶּחֱרַד כָּל־הָעָם אֲשֶׁר בַּמַּחֲנֶה׃

^19:16 And it came to pass on the third day, when it was morning, that there were thunders and lightnings and a thick cloud upon the mount, and the voice of a horn exceeding loud; and all the people that were in the camp trembled.

וַיּוֹצֵא מֹשֶׁה אֶת־הָעָם לִקְרַאת הָאֱלֹהִים מִן־הַמַּחֲנֶה וַיִּתְיַצְּבוּ בְּתַחְתִּית הָהָר׃

^19:17 And Moses brought forth the people out of the camp to meet God; and they stood at the nether part of the mount.

וְהַר סִינַי עָשַׁן כֻּלּוֹ מִפְּנֵי אֲשֶׁר יָרַד עָלָיו יְהוָה בָּאֵשׁ וַיַּעַל עֲשָׁנוֹ כְּעֶשֶׁן הַכִּבְשָׁן וַיֶּחֱרַד כָּל־הָהָר מְאֹד׃

^19:18 Now mount Sinai was altogether on smoke, because the LORD descended upon it in fire; and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.

וַיְהִי קוֹל הַשּׁוֹפָר הוֹלֵךְ וְחָזֵק מְאֹד מֹשֶׁה יְדַבֵּר וְהָאֱלֹהִים יַעֲנֶנּוּ בְקוֹל׃

^19:19 And when the voice of the horn waxed louder and louder, Moses spoke, and God answered him by a voice.

JPS 1917 and Tanach with Nikkud, public domain, via Sefaria

Shavuot celebrates receiving the Torah. On the holiday, the reading includes this scene: the morning of the third day at Mount Sinai.