Rashi on Numbers 15:3/רש"י על במדבר ט״ו:ג׳

1ועשיתם 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.

2לפלא 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. ...

3ריח ניחח [TO MAKE] A PLEASING ODOR — i.e. to cause satisfaction to Me.