Charles Ellicott Commentary Numbers 29:39

"These ye shall offer unto Jehovah in your set feasts, besides your vows, and your freewill-offerings, for your burnt-offerings, and for your meal-offerings, and for your drink-offerings, and for your peace-offerings." — Numbers 29:39 (ASV)

These things you shall do unto the Lord in your set feasts— Better, These sacrifices you shall offer to the Lord at your set seasons.

For your burnt offerings ...— The sacrifices prescribed in this chapter were appointed to be offered independently of all the burnt offerings, meal offerings, drink offerings, and peace offerings, which were made in performance of special vows, or as freewill offerings. (Numbers 15:1–13).