Both of my brother-in-laws, who work in the restaurant industry, are happy to work seven days a week. Imagining chopping vegetables for eight hours straight and then having to cover someone on my day off is downright appalling to me. But they are both of the opinion that a weekend (or other two-day break) breaks up their routine in a bad way. Another way of putting this is that Mondays are terrible precisely because you’ve had time off. You’re not used to working, and now you have to get used to it, and fast.
I really believe that working five days a week is best for me. But there are other ways of working it out–writing six or seven days a week, writing every other day, two days on/one day off . . . it’s worth experimenting, especially at the beginning of the dissertation writing process to find out what patterns keep you in your groove.
Another advantage of experimenting with your work habits is that you will confident that you’ve chosen the best process for you, which can be comforting when you discover other people’s habits. I usually have a reaction like “how could you possibly work that much?” and people tend to react to me like, “it’s amazing how you’re able to make so much time for things besides work!” As far as I can tell, there’s usually a bit of jealousy on both sides. But what other people do is not important. It’s only important that you aren’t ignorant of your own best practices.