וַיַּ֣עַשׂ יָרָבְעָ֣ם ׀ חָ֡ג בַּחֹ֣דֶשׁ הַשְּׁמִינִ֣י בַּחֲמִשָּֽׁה־עָשָׂר֩ י֨וֹם ׀ לַחֹ֜דֶשׁ כֶּחָ֣ג ׀ אֲשֶׁ֣ר בִּיהוּדָ֗ה וַיַּ֙עַל֙ עַל־הַמִּזְבֵּ֔חַ כֵּ֤ן עָשָׂה֙ בְּבֵֽית־אֵ֔ל לְזַבֵּ֖חַ לָעֲגָלִ֣ים אֲשֶׁר־עָשָׂ֑ה וְהֶעֱמִיד֙ בְּבֵ֣ית אֵ֔ל אֶת־כֹּהֲנֵ֥י הַבָּמ֖וֹת אֲשֶׁ֥ר עָשָֽׂה׃
Jerobeam setzte ein Fest ein im achten Monat, am fünfzehnten Tage des Monats, wie das Fest in Jehuda, [das aber am fünfzehnten des siebenten Monats gefeiert wurde], und opferte auf dem Altar; so machte er es auch in Bet-El, dass er den Kälbern opferte, die er gemacht, und stellte an in Bet-El die Priester der Höhen, die er errichtet.
Jerusalem Talmud Avodah Zarah
Rebbi Abin bar Cahana said, we find that Jeroboam also invented Sabbaths and holidays. That is what is written311K. 12:32–33.: Jeroboam made the holiday in the eighth month, on the fifteenth of the month, like the holiday in Jehudah, and went on the altar; so did he at Bethel to sacrifice … in the month which he invented. “In addition to” is written32The Qere is מִלִּיבּוּ “by his invention”; the Ketib is מִלְּבַד “in addition” (or “except”). This is read to mean that in addition to the holiday month which he invented, he invented something else. By comparing the expression used in Lev. 23:38, the “else” is found to be the Sabbath, justifying R. Abin bar Cahana’s statement. (Cf. Num. r. 21:23.), as you say, in addition to the Sabbaths of the Eternal.