I actually think Lisa Shearin covered this on her blog a few posts ago....but I'm saying what I feel.
I usually have Saturday as my day of writing. Even if, like this week, I haven't written for 4 days or so. Why? Because I need the break.
My muse still works away. She devours the books I read/games I play, and dictates what she wants to happen next in my wips.
Why take time out? Why not work all hours of the day to get my work finished? Because, I write about life experiences. I need to experience that in order to write about it. Plus, this is me. If i'm not careful about how much R&R (the original meaning needs to be used here) I receive, I can get sick (simple version), which means 0 writing for longer. However much fun I see it, anything active (writing, working on languages...plus anything which involves moving) uses energy which I use up far quicker than my enthusiasm.
So, since Saturday I have an appointment every morning, after which my brain is a little fried, I relax for the rest of the day.
Do you have a day off from writing?