In reply to Molly:
Just another perspective on the possibility/likelihood of comfortably fitting it into a single day. Steve Clegg and I did the route in October 2005; we were both already the wrong side of fifty, neither particularly fit, and encumbered with all the usual creaking joints, etc. On the day in question this was a hastily cobbled-together Plan B, Plan A being the Serenity/Sons combo. After a leisurely start we were sorting gear and racking up in the Ahwahnee Hotel parking lot. We were camping on a site out by the western park entrance, some miles beyond Crane Flat, so probably forty minutes drive away; at a guess it was sometime shortly after 10am. We noticed a group of three doing likewise nearby and wandered over to ask the obvious question; yes, they were Serenity-bound as well. Trying to beat them to the start seemed a bit underhand, and probably wouldn't have worked; they were almost ready to go, and looked rather younger than us. Not wishing a crowded day on any of us we had a quick think and decided that we could probably just about manage Snake Dike in the time available, so drove to Happy Isles and set off at about 11am.
The approach, as I recall, took three hours, putting us at the start at about 2pm. The roped climbing took just under two hours, followed by a further half-hour or so up the seemingly endless upper slabs to the top; that would have been at something approaching 5pm. We headed down apace and had got to somewhere around Nevada Fall when progress was slowed considerably by the onset of darkness; I can't recall exactly when that would have been at that time of year, but I think that another half hour before the necessary deployment of our solitary headtorch would have seen us onto easier terrain, or at least very close.
Don't get me wrong; we were obviously very comfortable with the climbing, we didn't get lost on the approach, and we didn't hang about any more than was dictated by advancing decrepitude. I would certainly advise starting your day in the knowledge that the route is your planned objective, leaving a bit earlier than we did, and taking a headtorch apiece; the Mist Trail is a very damp and slippy place when you can't quite see where you're going! But unless you find the actual climbing particularly challenging there's very little reason to have difficulty in fitting the experience into a single daylight cycle - particularly at this time of year with its much longer days.