וירא אהרן. שֶׁהָיָה בוֹ רוּחַ חַיִּים, שֶׁנֶּאֱמַר בְּתַבְנִית שׁוֹר אֹכֵל עֵשֶׂב (תהלים ק"ו), וְרָאָה שֶׁהִצְלִיחַ מַעֲשֵׂה שָׂטָן וְלֹא הָיָה לוֹ פֶּה לִדְחוֹתָם לְגַמְרֵי: וירא אהרן AND WHEN AARON SAW that there was the breath of life in it — as it is said with reference to the golden calf, (Psalms 106:20) “[They changed their glory] into a similitude of an ox that ate grass” — and he realized that Satan’s work had succeeded and that he had no argument (lit., mouth) to put them (the people) entirely off.
ויבן מזבח. לִדְחוֹתָם: ויבן מזבח HE BUILDED AN ALTAR to put them off.
ויקרא חג לה' מחר. וְלֹא הַיּוֹם, שֶׁמָּא יָבֹא מֹשֶׁה קֹדֶם שֶׁיַּעַבְדוּהוּ, זֶהוּ פְּשׁוּטוֹ; וּמִדְרָשׁוֹ, בְּוַיִּקְרָא רַבָּה, דְּבָרִים הַרְבֵּה רָאָה אַהֲרֹן – רָאָה חוּר בֶּן אֲחוֹתוֹ שֶׁהָיָה מוֹכִיחָם וַהֲרָגוּהוּ, וְזֶהוּ וַיִּבֶן מִזְבֵּחַ לְפָנָיו – וַיָּבֶן מִזָּבוּחַ לְפָנָיו – וְעוֹד רָאָה וְאָמַר מוּטָב שֶׁיִּתָּלֶה בִי הַסֵּרָחוֹן וְלֹא בָהֶם, וְעוֹד רָאָה וְאָמַר אִם הֵם בּוֹנִים אֶת הַמִּזְבֵּחַ, זֶה מֵבִיא צְרוֹר וְזֶה מֵבִיא אֶבֶן וְנִמְצֵאת מְלַאכְתָּן עָשׂוּי בְּבַת אַחַת, מִתּוֹךְ שֶׁאֲנִי בוֹנֶה אוֹתוֹ וַאֲנִי מִתְעַצֵּל בִּמְלַאכְתִּי, בֵּין כָּךְ וּבֵין כָּךְ מֹשֶׁה בָא: ויקרא חג לה׳ מחר AND CALLED OUT … TO-MORROW IS A FESTIVAL TO THE LORD — to-morrow, not to-day, for he hoped that Moses might return before they would worship it (the calf). This is the plain meaning of the verse. The Midrashic explanation of it in Leviticus Rabbah 10:3 (— it supplies an object to the verb וירא —) is: Aaron saw many things; he saw Hur, his sister’s son, who had reprimanded them, and whom they had killed. This is the meaning of ויבן מזבח, viz., וַיָבֶן, he realized (taking the word as from the root בון, to understand, to realize) מִזָּבוּחַ לפניו from him who lay slaughtered before him (מזבח is vowelled to be read as מִזָּבוּחַ) what would happen to him if he offered resistance. A further explanation of וירא אהרן in the Midrash is: He saw what the situation was and said: It is better that the offence should attach itself to me than to them. And yet a further Midrashic explanation of וירא is: He looked into the matter and said: If they build this altar themselves, one will bring a clod and another a stone and the result will be that their work will be accomplished all at once; through myself building it and being dilatory in my work, in the meantime Moses may come.
חג לה'. בְּלִבּוֹ הָיָה לַשָּׁמַיִם, בָּטוּחַ הָיָה שֶׁיָּבֹא מֹשֶׁה וְיַעַבְדוּ אֶת הַמָּקוֹם: חג לה׳ A FESTIVAL TO THE LORD — not to the golden calf. In his heart it (the feast) was for Heaven (the Lord). He felt confident that Moses would return by the morrow and that they would worship the Omnipresent (Leviticus Rabbah 10:3).