I have a set of mavic rims with an 8 speed freewheel that I have wanted to put on my older bike. My bike has a 7 speed freewheel on it now. Can I put the 7 speed on the 8 speed hub? Probably a dumb question, but I will only have it once!

I'm assuming you mean it is a 6 speed freehub? If so yes you can fit a 7 speed cassette on it just need to add a spacer.

I think Kev & Daddy mean 8-spd freehub. His question is would a 7-spd cassette cassette fit on the 8-spd freehub. I would imagine if the splines are compatible, all you need is a spacer..... assuming these are Shimano compatible components.
Or....are we talking freewheels on a threaded hub?

I put a 7spd cassette on a 8spd hub. it just needed a spacer. I think it went on before the cassette was installed. I just had it apart last week but I can't remember which order it went back together. Check your LSB for a spacer.

8-speed freewheels are really quite rare. do you mean a freewheel, or a freehub?

If it is either, the answer is yes, probably, with a spacer. If you have indexed shifters, though, it may or may not work. Also, since 7-speed usually uses 126mm dropout spacing, and 8-speed uses 130 mm dropouts, you might have issues with fit.