This is a topic that I struggle with. I can understand the idea behind trying to write every day but sometimes it seems a little more than necessary. Even if we consider writing to be our jobs, most jobs aren't 7 days a week. Having one day off can have a positive affect on a person's writing. However, in McAleer's book, all the authors quoted say to write every single day.
S. J. Rozan
"Write every day. Every day. But not like you're on a diet, where as soon as you goof up, you figure yourself for a miserable failure and give up, and you do this three times a year, with new and useless resolve each time.
No, write every day the way an athlete practices a sport, or a musician an instrument. You're doing the same thing: not just producing pages in your manuscript, but even more important, keeping fit, keeping toned, keeping in practice. Do this for the rest of your life. Or at least, the rest of your writer life."
I do believe in having realistic writing goals, but for me: I am not sure if I'm at a place where I can write on novels every single day. Sometimes I do a poem one day, others work on a story. But there are times when I don't write. Then again, I'm just starting out in this business. Maybe in the near future I'll post saying everyone should write every day.
For now, I agree with Mur Lafferty. Do what works best for you.