Our iterations are 1 week in duration and they start on Wednesday and finish on Tuesday. Friday night is the start of the weekend so it's great to always get away on time. And coming in on the Monday with energy levels boosted by the weekend's rest, you're ready for the push towards the iteration review on Tuesday.
Compare this to approaching the iteration review on Friday with depleted energy levels because you've worked a contiguous week on the iteration without a break. And then having to work late on Friday to get the iteration done. There's a good chance you'll be too exhausted to enjoy your Saturday.
I know which I prefer. I like starting iterations on Wednesdays.