Tafsiran Bilangan 15:3: Rashi, Sforno, Siftei Chakhamim & Rav Hirsch

ועשיתם אשה — This is not a command: when you come into the land you shall make a fire-offering — but the meaning is: When you will come there and it enters your mind to make a fire-offering unto the Lord.

לעשות ריח ניחוח.....והקריב המקריב, until the advent of the sin of the golden calf, the expression ריח ניחוח had not surfaced as no libations plus gift offerings had been needed in order to make the offerings truly pleasing in the eyes of the Lord. This is why we do not find such an expression in connection with the offering of Hevel (Genesis 4,4, of Noach Genesis 8,21, and of Avraham in Genesis 12,7) Neither do we find it in Exodus 24,5 where the burnt offerings and meat offerings offered by the firstborns (before they had been exchanged for the tribe of Levi) were described. As of the sin of the golden calf gift offerings and libations were required to make public offerings pleasing to the Lord, i.e. לריח ניחוח. After the sin of the spies even offerings by private individuals had to be accompanied by such libations and gift offerings, מנחה ונסכים, in order to qualify for the expression ריח ניחוח as proving that the offering had pleased the Lord.

This does not denote a command. Meaning that if this were a command, it would not be correct to say “by verbalizing as a pledge or as a donation” because donations are not commanded. Thus Rashi explains that “when it occurs to you…”

לפלא נדר או בנדבה וגו׳ TO DISTINGUISH YOURSELVES BY A VOW OR A FREE-WILL OFFERING [OR ON YOUR FESTIVALS] — i.e. or that you would make the fire-offering on account of the obligatory festival sacrifice which I have obligated you to offer on a festival. ...

ריח ניחח [TO MAKE] A PLEASING ODOR — i.e. to cause satisfaction to Me.