How do I taper for a century+ ride?
I would agree that a short ride on Saturday is useful to keep things ticking over, resting on the Friday, so long as the substitute commute isn't too stressful, might be more useful. Your body needs some down time and a 'clean' 24+ hours is quite important for genuine recovery.  

The thing to do on the Saturday is to use the short ride as a dress rehearsal, use all the kit that you plan to take with you - you've probably got more time to do this on the Saturday. 

So, if you're going to drive to the start line, then do that on the Saturday too. At least, pack the bike and bits into/on to the car and drive somewhere to do the test ride. Do your best to mimic game day. Just to let you relax knowing you haven't forgotten anything. This is especially important if there's anything new in your setup - nothing new on the day is a crucial mantra (not new shoes, socks, tyre, nothing!).