John Gill Commentary 1 Chronicles 23:31

"and to offer all burnt-offerings unto Jehovah, on the sabbaths, on the new moons, and on the set feasts, in number according to the ordinance concerning them, continually before Jehovah;" — 1 Chronicles 23:31 (ASV)

And to offer all burnt sacrifices unto the Lord
This was the work of the priests to offer such sacrifices, and not the Levites; but the meaning is, according to the Targum, and so Kimchi, that these not only sung the praises of God morning and evening, but at all times when burnt offerings were offered to the Lord; besides, some of them helped the priests at such times in slaying the sacrifices, and bringing them to the altar: and especially their service was requisite

in the sabbaths, in the new moons, and on the set feasts, by number ;
because, besides the daily sacrifices, there were additional offerings at those times:

according to the order commanded unto them continually before the Lord ;
see (Numbers 28:1–29:40) .