This posting--late at night, just before the day expires--is mostly just so that I can keep up my string of continuously posting on my blog.
But this is a principle that helps with any long-term project: make an effort to work on it every day--even if it is just a little bit. That little bit on the day when you don't really feel like you're into it, or feel like you're tired or too busy or whatever, that little bit plays a crucial role in helping build the habit of working on your project.
If you've been away from your dissertation--because you took time off, or just because you've been procrastinating so darn much--it can be hard to get back into it, because you're not used to using your time in that way. Making the commitment to do something--anything--on a frequent and regular basis (and I don't mean once a week) helps you realign your habits to get to work on the project.
I suppose it's possible to finish a dissertation working on it one day a week. I wouldn't bet on it being the right strategy for timely completion.
So work on your project every day, even if all you do is write something brief about it more for the purpose of saying you did something than in the hope that you get a lot of progress out of the effort.
This principle of working on your project every day is expressed in Jean Bolker's book "Writing Your Dissertation in Fifteen Minutes a Day". The reasoning I present is not exactly like hers, but that difference makes her book a worthy read. If you follow her advice in that book, your path to completing your dissertation will be considerably smoother.