וַיַּ֜עַל עַֽל־הַמִּזְבֵּ֣חַ ׀ אֲשֶׁר־עָשָׂ֣ה בְּבֵֽית־אֵ֗ל בַּחֲמִשָּׁ֨ה עָשָׂ֥ר יוֹם֙ בַּחֹ֣דֶשׁ הַשְּׁמִינִ֔י בַּחֹ֖דֶשׁ אֲשֶׁר־בָּדָ֣א מלבד [מִלִּבּ֑וֹ] וַיַּ֤עַשׂ חָג֙ לִבְנֵ֣י יִשְׂרָאֵ֔ל וַיַּ֥עַל עַל־הַמִּזְבֵּ֖חַ לְהַקְטִֽיר׃ (פ)
And he went up unto the altar which he had made in Beth-el on the fifteenth day in the eighth month, even in the month which he had devised of his own heart; and he ordained a feast for the children of Israel, and went up unto the altar, to offer.
Rashi on I Kings
Of the eighth month. He expounded to them that this was the month of the harvest, and was, therefore, the proper time for the festival to be.
Rashi on I Kings
He thought up out of his own heart. An expression of falsehood, controver, in O.F.