I think as we get older our personal routines take on a life of their own. They become who we are, what we are.
"Oh", she says, "I can't come Wednesday, that's the day I have to bake my cookies". Even though there may be six perfectly good unused days left, and the cookies take an hour out of her morning, Wednesday is the day she bakes cookies. It may also be an easy excuse used too often, when she wants to get out of shopping, or visiting a neighbor she really doesnt like, or just because.
I use Tuesdays ("oh," I say, regretfully. "I can't make it on Tuesday, that's the day I volunteer.") And even if I don't go, it's still the day I volunteer.