Well, one way to choose is to figure out which would end up with a better day off for you, assuming it would end up with a day off.

Another thing to contemplate is when other blogs tend to not post. That can be applied two ways: post preferentially on days they don't because you think people might be eager for reading material, or don't post on those days because you think people might be not expecting to have anything to read on those days & therefore be more likely to skip looking for it.

I'm very good at not actually answering questions.