The opening of the middle berakhah of Shabbos Morning Admidah is somewhat famously odd, “Moshe will be rejoicing with the giving of his portion, for You called him a loyal servant. A crown of splendor descended on his head when he stood before You in Mount Sinai, and two stone luchos he brought down in his hand. And Shabbos was written in them, as also it says in your Torah:”
What’s this focus on Moshe’s joy? And why “Moshe will be rejoicing” — bringing down the luchos happened already! (My other proposed explanation of the not-really-fuuture tense of the verb “yismach” aside for the moment.)
And then what do we quote after “and so it says”? Not the commandment of Shabbos as it appears in either version of the luchos. Instead, we bring down a quote from parashas Ki-Sisa (31:16-17):
וְשָׁמְרוּ בְנֵי־יִשְׂרָאֵל אֶת־הַשַּׁבָּת לַעֲשׂוֹת אֶת־הַשַּׁבָּת לְדֹרֹתָם בְּרִית עוֹלָם׃ בֵּינִי וּבֵין בְּנֵי יִשְׂרָאֵל אוֹת הִוא לְעֹלָם כִּי־שֵׁשֶׁת יָמִים עָשָׂה יְהוָה אֶת־הַשָּׁמַיִם וְאֶת־הָאָרֶץ וּבַיּוֹם הַשְּׁבִיעִי שָׁבַת וַיִּנָּפַשׁ׃
The Jewish People shall keep the Shabbos, observing the Shabbos throughout their generations as an eternal covenant. It shall be an eternal sign between Me and the Jewish People that in six days Hashem made heaven and earth, and on the seventh day He stopped and “Refreshed.”
I think an answer lies in the following verses (v. 18, 32:1):
וַיִּתֵּן אֶל־מֹשֶׁה כְּכַלֹּתוֹ לְדַבֵּר אִתּוֹ בְּהַר סִינַי שְׁנֵי לֻחֹת הָעֵדֻת לֻחֹת אֶבֶן כְּתֻבִים בְּאֶצְבַּע אֱלֹקים׃
He gave to Moshe, when He finished speaking with him on Mount Sinai, the two luchos of testimony, stone luchos inscribed by the “Finger” of G-d.
וַיַּרְא הָעָם כִּי־בֹשֵׁשׁ מֹשֶׁה לָרֶדֶת מִן־הָהָר וַיִּקָּהֵל הָעָם עַל־אַהֲרֹן וַיֹּאמְרוּ אֵלָיו קוּם עֲשֵׂה־לָנוּ אֱלֹהִים אֲשֶׁר יֵלְכוּ לְפָנֵינוּ…
The nation saw that Moshe was tarrying to come down from the mountain, and the nation ganged up against Aharaon sand said to him, “Get up, make for us gods who will go before us…”
When does Hashem tell Moshe the “VeShomru” version of the commandment to keep Shabbos? Immediately before sending him down to deal with the Eigel haZahav.
In the Shabbos morning davening we draw a picture of Moshe descending the moutain — but given that we’re introducing VeShomru, it appears the “two stone luchos he brought down in his hand” were the first ones.
And this could well be reflected in the text of VeShomru. Notice that in these two verses, Hashem refers to the covenant being eternal three times — we will keep Shabbos “ledorosam — for all their generations” as a “beris olam – eternal covenant”, and “os hi le’olam — it shall be an eternal sign.)
What VeShomru tells Moshe is that no matter what he is about to see when he descends the mountain, it won’t end up abrogating the beris between Hashem and the Jewish People. That is guaranteed to be eternal.
And since Moshe was told that his portion was to initiate an eternal covenant between Hashem and the Jewish People, his joy too is never-ending. Moshe couldn’t have been happy at the time of his descent, but he was assured that he will in the future be happy, and forever.