In the Maftir reading for Shabbat HaChodesh, which comes from the fourth aliyah of Parashat Bo, we encounter the required perpetual observance of Pesach, including the following injunction in Ex. 12:17:
(יז) וּשְׁמַרְתֶּם֮ אֶת־הַמַּצּוֹת֒ כִּ֗י בְּעֶ֙צֶם֙ הַיּ֣וֹם הַזֶּ֔ה הוֹצֵ֥אתִי אֶת־צִבְאוֹתֵיכֶ֖ם מֵאֶ֣רֶץ מִצְרָ֑יִם וּשְׁמַרְתֶּ֞ם אֶת־הַיּ֥וֹם הַזֶּ֛ה לְדֹרֹתֵיכֶ֖ם חֻקַּ֥ת עוֹלָֽם׃
(17) You shall observe the [Feast of] Unleavened Bread, for on this very day I brought your ranks out of the land of Egypt; you shall observe this day throughout the ages as an institution for all time.
The above translation of וּשְׁמַרְתֶּם אֶת־הַמַּצּוֹת is not exactly literal: "You shall observe the [Feast of] Unleavened Bread". As shown in the bracketed text, it contains an implicit Feast, like the explicit Feast (חַג) in the beginning of Ex. 23:15, which uses the same verb (לִשְׁמֹר).
(טו) אֶת־חַ֣ג הַמַּצּוֹת֮ תִּשְׁמֹר֒ שִׁבְעַ֣ת יָמִים֩ תֹּאכַ֨ל מַצּ֜וֹת כַּֽאֲשֶׁ֣ר צִוִּיתִ֗ךָ לְמוֹעֵד֙ חֹ֣דֶשׁ הָֽאָבִ֔יב כִּי־ב֖וֹ יָצָ֣אתָ מִמִּצְרָ֑יִם וְלֹא־יֵרָא֥וּ פָנַ֖י רֵיקָֽם׃
(15) You shall observe the Feast of Unleavened Bread—eating unleavened bread for seven days as I have commanded you—at the set time in the month of Abib, for in it you went forth from Egypt; and none shall appear before Me empty-handed;
Other translations of Ex. 12:17 are literal (e.g., The Chumash: The Stone Edition): "You shall safeguard the matzos".
Interestingly, between Ex. 12:17 and Ex. 23:15, there is a nearly identical phrase in a completely different context, Aseret HaDibrot (The Ten Commandments), in Ex. 20:6:
(ו) וְעֹ֥֤שֶׂה חֶ֖֙סֶד֙ לַאֲלָפִ֑֔ים לְאֹהֲבַ֖י וּלְשֹׁמְרֵ֥י מִצְוֺתָֽי׃ {ס}
(6) but showing kindness to the thousandth generation of those who love Me and keep My commandments.
Here, וּלְשֹׁמְרֵי מִצְוֺתָי is strikingly similar to וּשְׁמַרְתֶּם אֶת־הַמַּצּוֹת in Ex. 12:17, especially if their vowels are ignored: ולשמרי מצותי andושמרתם את־המצות. This particular construction (לִשְׁמֹר מִצְוֹת) is repeated elsewhere in the Torah (e.g., Lev. 22.31, Deut. 4:2, and Deut. 6:17).
How do we know that the last word of Ex. 20:6 is not מַּצּוֹתָֽי (My matzot), instead of מִצְוֺתָֽי (My commandments)? We are literally commanded to "safeguard the matzos", so this alternative also makes sense. The latter's (non-possessive) singular form, מִצְוָה, does not appear in the Torah at all (it does in Prov. 13:13 and Eccl. 8:5), and neither does its "proper" plural with a vav and a cholam malei, מִצְווֹת (it does in one rendering of Ezra 7:11, apparently the only occurrence in the Tanakh). In contrast, the singular form of מַצּוֹת (as well as מַצֹּת), מַצָּה, does appear in the Torah (e.g., see Lev. 2:4-5). It may be that the earlier usage in Ex. 16:28--
(כח) וַיֹּ֥אמֶר יְהֹוָ֖ה אֶל־מֹשֶׁ֑ה עַד־אָ֙נָה֙ מֵֽאַנְתֶּ֔ם לִשְׁמֹ֥ר מִצְוֺתַ֖י וְתוֹרֹתָֽי׃
(28) And יהוה said to Moses, “How long will you all refuse to obey My commandments and My teachings?
--causes us to understand the last word of Ex. 20:6 to be מִצְוֺתָֽי.
What we have then is the following: מַצּוֹת associated with Pesach and מִצְוֹת associated with Shavu'ot, being in our tradition זְמַן מַתַּן תּוֹרָתֵנוּ (The Season of the Giving of our Torah), identified as the time of the giving of Aseret HaDibrot (The Ten Commandments).
In the Torah, Shavu'ot is not associated with זְמַן מַתַּן תּוֹרָתֵנוּ, but rather with the wheat harvest, as indicated in the first part of Ex. 34:22:
(כב) וְחַ֤ג שָׁבֻעֹת֙ תַּעֲשֶׂ֣ה לְךָ֔ בִּכּוּרֵ֖י קְצִ֣יר חִטִּ֑ים וְחַג֙ הָֽאָסִ֔יף תְּקוּפַ֖ת הַשָּׁנָֽה׃
(22) You shall observe the Feast of Weeks, of the first fruits of the wheat harvest; and the Feast of Ingathering at the turn of the year.
The Torah connects Pesach and Shavu'ot in relation to each other in Lev. 23:15-16 and in Deut. 16:8-10:
(טו) וּסְפַרְתֶּ֤ם לָכֶם֙ מִמׇּחֳרַ֣ת הַשַּׁבָּ֔ת מִיּוֹם֙ הֲבִ֣יאֲכֶ֔ם אֶת־עֹ֖מֶר הַתְּנוּפָ֑ה שֶׁ֥בַע שַׁבָּת֖וֹת תְּמִימֹ֥ת תִּהְיֶֽינָה׃ (טז) עַ֣ד מִֽמׇּחֳרַ֤ת הַשַּׁבָּת֙ הַשְּׁבִיעִ֔ת תִּסְפְּר֖וּ חֲמִשִּׁ֣ים י֑וֹם וְהִקְרַבְתֶּ֛ם מִנְחָ֥ה חֲדָשָׁ֖ה לַיהֹוָֽה׃
(15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to יהוה.
(ח) שֵׁ֥שֶׁת יָמִ֖ים תֹּאכַ֣ל מַצּ֑וֹת וּבַיּ֣וֹם הַשְּׁבִיעִ֗י עֲצֶ֙רֶת֙ לַיקוק אֱלֹקֶ֔יךָ לֹ֥א תַעֲשֶׂ֖ה מְלָאכָֽה׃ {ס} (ט) שִׁבְעָ֥ה שָׁבֻעֹ֖ת תִּסְפׇּר־לָ֑ךְ מֵהָחֵ֤ל חֶרְמֵשׁ֙ בַּקָּמָ֔ה תָּחֵ֣ל לִסְפֹּ֔ר שִׁבְעָ֖ה שָׁבֻעֽוֹת׃ (י) וְעָשִׂ֜יתָ חַ֤ג שָׁבֻעוֹת֙ לַיקוק אֱלֹקֶ֔יךָ מִסַּ֛ת נִדְבַ֥ת יָדְךָ֖ אֲשֶׁ֣ר תִּתֵּ֑ן כַּאֲשֶׁ֥ר יְבָרֶכְךָ֖ יקוק אֱלֹקֶֽיךָ׃
(8) After eating unleavened bread six days, you shall hold a solemn gathering for your God יקוק on the seventh day: you shall do no work. (9) You shall count off seven weeks; start to count the seven weeks when the sickle is first put to the standing grain. (10) Then you shall observe the Feast of Weeks for your God יקוק, offering your freewill contribution according as your God יקוק has blessed you.
As we count--seven weeks starting the second day of Pesach--Shavu'ot begins 50 days after we leave our חָמֵץ (leaven). Apparently, it takes 50 days for מַצּוֹת to be transformed into מִצְוֹת.
Finally, there seems to be a good practical reason why Shavu'ot is not known as חַג הַמִצְוֹת (The Feast of the Commandments) instead of חַג הַשָּׁבֻעוֹת. Deut. 16:16-17 identifies the three occasions and the place each year that every male is required to make an appropriate offering:
(טז) שָׁל֣וֹשׁ פְּעָמִ֣ים ׀ בַּשָּׁנָ֡ה יֵרָאֶ֨ה כׇל־זְכוּרְךָ֜ אֶת־פְּנֵ֣י ׀ יקוק אֱלֹקֶ֗יךָ בַּמָּקוֹם֙ אֲשֶׁ֣ר יִבְחָ֔ר בְּחַ֧ג הַמַּצּ֛וֹת וּבְחַ֥ג הַשָּׁבֻע֖וֹת וּבְחַ֣ג הַסֻּכּ֑וֹת וְלֹ֧א יֵרָאֶ֛ה אֶת־פְּנֵ֥י יקוק רֵיקָֽם׃ (יז) אִ֖ישׁ כְּמַתְּנַ֣ת יָד֑וֹ כְּבִרְכַּ֛ת יקוק אֱלֹקֶ֖יךָ אֲשֶׁ֥ר נָֽתַן־לָֽךְ׃ {ס}
(16) Three times a year—on the Feast of Unleavened Bread, on the Feast of Weeks, and on the Feast of Booths—all your males shall appear before your God יקוק in the place that [God] will choose. They shall not appear before יקוק empty-handed, (17) but each with his own gift, according to the blessing that your God יקוק has bestowed upon you.
This requirement would be difficult to understand if the three occasions
in Deut. 16:16 were presented as follows instead (without vowels or trop): בחג המצות ובחג המצות ובחג הסכות.