And to offer all burnt sacrifices unto the LORD in the sabbaths, in the new moons, and on the set feasts, by number, according to the order commanded unto them, continually before the LORD:

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;

As well in the oblation of the holocausts of the Lord, as in the sabbaths and in the new moons, and the rest of the solemnities, according to the number and ceremonies prescribed for every thing, continually before the Lord.

and to offer all burnt offerings unto the LORD, in the sabbaths, in the new moons, and on the set feast, in number according to the ordinance concerning them, continually before the LORD:

and for all the burnt-offerings, burnt-offerings to Jehovah for sabbaths, for new moons, and for appointed seasons, by number, according to the ordinance upon them continually, before Jehovah.

At every offering of burned offerings to the Lord, on Sabbaths, and at the new moons, and on the regular feasts, in the number ordered by the law, at all times before the Lord;

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;

and to offer all burnt offerings to the LORD, on the Sabbaths, on the new moons, and on the set feasts, in number according to the ordinance concerning them, continually before the LORD;

as much in the oblation of the holocausts of the Lord, as in the Sabbaths and new moons and other solemnities, according to the number and ceremonies for each and every matter, perpetually before the Lord.

Though the Levites were not allowed by themselves to offer sacrifice, yet there were many respects in which they assisted the priests when sacrifice was offered. See 2-Chronicles 29:34; 2-Chronicles 35:11-12.
The set feasts - The Passover, Feast of Pentecost, and Feast of Tabernacles (marginal reference).

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.

"And for all the bringing of offerings to Jahve on sabbaths, the new moons, and the feasts, in the number according to the law concerning them (i.e., according to the regulations that existed for this matter), continually before Jahve." It was the duty of the Levites to procure the necessary number of beasts for sacrifice, to see to their suitableness, to slaughter and skin them, etc. תּמיד refers to עלות, the burnt-offerings for Jahve, which are תּמיד, because they must always be offered anew on the appointed days.