When is Cambridge busiest in late summer? Sunday would be best but if mid-week is less crowded we have some flexibility.
We must have been there mid-week in early July a couple years ago. It wasn't too crowded for us. In fact, we were there right after the Tour de France went through town. We walked all around and visited Kings College Chapel. If you want to go to that, Sunday might be a problem. I'm sure Rick Steves' book (which I no longer have) offers suggestions on that.
Saturday will be the busiest day as that is when most people will be shopping in town. Parking may be more of a problem that day. There are several park and rides, although parking is free, the bus is quite expensive and will be more crowded on a saturday. Sunday would be a better day, but you would have to check to see if the colleges are open. During the week, you could park closer in town. There will be several college students working on the punting boats that will be walking around trying to get you to take a ride. The punting is fun and worth it. You get a history tour and the buildings from the river are beautiful. I took my teenagers there last year and Cambridge was one of their favorite places.
Thanks to both of you for the feedback. My RS Great Britain guide is a bit old (2011), but it looks like Sunday won't be a problem for King's College Chapel (open 10:00-17:00 during breaks). Unfortunately, Wren Library will be closed which is a deal-breaker for my wife. We'll go during the week instead and plan to spend a full day which will include Punting on the Cam!