Leviticus 6:9

This is the law of the burnt-offering - This law properly refers to that burnt-offering which was daily made in what was termed the morning and evening sacrifice; and as he had explained the nature of this burnt-offering in general, with its necessary ceremonies, as far as the persons who brought them were concerned, he now takes up the same in relation to the priests who were to receive them from the hands of the offerer, and present them to the Lord on the altar of burnt-offerings.

Because of the burning upon the altar all night - If the burnt-offering were put all upon the fire at once, it could not be burning all night. We may therefore reasonably conclude that the priests sat up by turns the whole night, and fed the fire with portions of this offering till the whole was consumed, which they would take care to lengthen out till the time of the morning sacrifice. The same we may suppose was done with the morning sacrifice; it was also consumed by piecemeal through the whole day, till the time of offering the evening sacrifice. Thus there was a continual offering by fire unto the Lord; and hence in Leviticus 6:13 it is said: The fire shall ever be burning upon the altar, it shall never go out. If at any time any extraordinary offerings were to be made, the daily sacrifice was consumed more speedily, in order to make room for such extra offerings. See more on this subject in Clarke's note on Leviticus 6:23 (note). The Hebrew doctors teach that no sacrifice was ever offered in the morning before the morning sacrifice; and none, the passover excepted, ever offered in the evening after the evening sacrifice; for all sacrifices were made by day-light. The fat seems to have been chiefly burned in the night season, for the greater light and convenience of keeping the fire alive, which could not be so easily done in the night as in the day time.

Rather, "This, the burnt-offering, shall be upon the fire on the altar all night unto the morning." See Exodus 29:38-46, with the notes.

6:9 And the Lord spake - Hitherto he hath prescribed the sacrifices themselves; now he comes to the manner of them. The burnt - offering - The daily one, which 29:38 , Num 28:3, as the following words shew.This was to be so managed and laid on piece after piece, that the fire might be constantly maintained by it. The morning burnt - offerings were to be kept burning all the day from morning to night also; but he mentions not that, because there was such a constant succession of sacrifices in the day - time that there needed no law for feeding and keeping in the fire then; the only danger was for the night, when other sacrifices were not offered, but only the evening burnt - offering, which if it had been consumed quickly, as the morning burnt - offering was, there had been danger of the going out of that fire, which they were commanded diligently and constantly to keep in.