1. Stephen King says you should, and there's a pretty good chance he knows what he's talking about. He doesn't give anybody a day off, though. I found the day off necessary. So I gave it to myself.
2. You will actually be a writer. When you ask yourself what it is that you do here on earth, what your job is, you can answer with that one word, "writer," and be sure you are telling yourself the truth. You're not trying to write. You're not hoping to get around to writing when the inspiration hits you. You're not thinking about writing. You're a writer. A writer writes, whether or not the writer "feels like it" at the moment. An unwriter doesn't write. Waiting until we feel like it is a game we play. Writer's block is a game that we play.
3. You can set the rules up however you want. I make the rules very lenient. Things like blog posts can count if I want them to. Terrible first drafts I would never show anyone, even under torture, can count if I want them to. Backstory scenes I know will never make the book can count if I want them to. My own rules are: The main draft of the novel has to move forward in a way that adds to its word count; the same text can count for the next day if I edit it heavily enough, and yes, I am the one who decides what "heavily enough" means; blog posts count because they sometimes inform the content of the novel, but they can't be the only thing I write that day.
4. You will establish a personal routine of writing six days out of seven, which will bring shape and purpose to your life. Whatever else is happening, whatever is going well or poorly, the writing is happening. That will be good for you and good for those with whom you interact.
5. You will learn to edit your own material, or get better at it if you are already doing that. By generating a lot of text, more than you could use for your story, you will get personal experience with deleting things that don't work.
6. You will create more clarity on your own personal issues. It doesn't sound like that's what you're out to achieve here, but it happens, whether you mean it to or not, and once it begins happening regularly, you will begin to wonder why everyone doesn't write a thousand words a day. We've all got problems. If you write a lot, you will eventually get a more profound understanding of what your problems really are, and begin to develop an understanding of what the best ways to deal with them are -- and aren't.
7. You will stumble upon magnificent ideas. Often, they will come in the space between words 900 and 1000. Don't ask me why. They just do. I think they must have a sense of humor.
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