Midrash on I Chronicles 23:31: Bereshit Rabbah & Classical Sources

וּלְכֹ֨ל הַעֲל֤וֹת עֹלוֹת֙ לַיהוָ֔ה לַשַּׁבָּת֔וֹת לֶחֳדָשִׁ֖ים וְלַמֹּעֲדִ֑ים בְּמִסְפָּ֨ר כְּמִשְׁפָּ֧ט עֲלֵיהֶ֛ם תָּמִ֖יד לִפְנֵ֥י יְהוָֽה׃

and to offer all burnt-offerings unto the LORD, on the sabbaths, on the new moons, and in the appointed seasons, in number according to the ordinance concerning them, continually, before the LORD;