| | why "day"
I think part of the problem is calling it a low motivation day. It implies that once you notice this is it one of "those" days, you know you're doomed for the entire day. In my experience, at some point I realized that this was some kind of underlying belief which was incorrect, and that it was letting my laziness/procrastination take over until something external would kick me out of that state.
Nowadays I don't do that anymore. I still get days when I wake up unmotivated, but I'm much more aware, I notice the state I'm in and accept that it's just a temporary state. Some of the advice given before makes sense to me, especially
listening to or reading self-help/motivational material -- I find it a really good solution, because it's really hard for me to concentrate (e.g. meditating, writing) when I'm not motivated.