I wish I had a buck for every time I've heard "a writer writes every day."
Or to put it another way, a truly committed, professional (or potentially professional) writer proves his or her dedication by writing every single day. Otherwise, you're just a hobby writer. A wanna-be. One of the thousands who say, "I'd love to write a book some day," but never do.
Let me confess this here and now: I have been a published author for twenty years AND I DO NOT WRITE EVERY SINGLE DAY.
There. I've said it.
In fact, I've just taken a few days off from working on my manuscript. I had a lot of things to do last week and (if you follow me on Twitter, you'll know this already) I was stuck with my story. I felt absolutely bogged down, not sure if what I was writing was what was needed at that particular point, as well as trying to figure out how to add time into the tale.
So I gave myself permission to take time off from writing. I had some appointments and meetings and I took care of some administrative and household tasks I'd been putting off. I did all those things without guilt and it felt great. I stopped fretting about the manuscript. That felt great, too.
And best of all, when I got back to the writing yesterday, I was refreshed, I was energized, I had ideas about what to do at the sticky bits, and I wasn't haunted by all those other obligations and tasks that had been in the back of my mind.
Now, I do understand the basic notion behind people saying "a writer writes every day." Writing is not easy. If you're not dedicated, you might take a break that winds up being permanent.
Writing every day keeps your story and characters fresh in your mind, and keeps those writing muscles honed.
However, taking a complete break was what I needed to do, to give my full attention to what else was going on in my life, to clear those decks so I could get back to the writing without guilt and with fresh eyes.
So I say if you can't or don't write every single day, that doesn't necessarily make you any less of a "real" or dedicated writer. It simply makes you a writer who doesn't write every day.