We have a choice of spending either Friday or Saturday in May in Munich. Which day is the best to be in Munich?

I've been in Munich many times on Friday and Saturday. I don't think there will be much difference except more activity at the Marienplatz, more school age kids on Saturday. It's fun there no matter which day you choose.

Since most stores remain open on Saturday now, it really doesn't make much difference which day you pick.

HI scatcat and treplow - Thanks for your replies. We will schedule our day in Munich around the weather in the mountains.

The only thing I would add is if you are driving into Munich, traffic on a Friday evening is not good.

norrisken - Thanks for your reply. I should have put in my post that we are taking the train into Munich

You may be interested in some of my maps I've created replaying some tours I've done.
http://www.thirdmansystems.com/vacation/maps/maps.htm
I have to recommend a book that covers many smaller interesting stops in Germany/Bavaria. "Germany's Best-Loved Driving Tours" Newest edition is 7th I believe. Sounds like you have a great trip planned. I only wish I was headed there soon!