Albert Barnes 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)

Although the Levites were not allowed to offer sacrifices by themselves, they still assisted the priests in many ways when a sacrifice was offered (see 2 Chronicles 29:34; 2 Chronicles 35:11–12).

The appointed feasts: The Passover, the Feast of Pentecost, and the Feast of Tabernacles (as noted in the margin).