ohhh I can relate! "sad and unmotivated" is the perfect phrase to describe it, and one day is about my limit too.
I like doing things where I at least get out in the world and SEE other people, even if they are strangers. running errands (if there are any that need to be run, don't just go to Target and spend money! LOL) or going for a run or walking my dogs... the latter two have the added benefit of endorphins, and being totally free! leisurely bike rides are great too!
if you aren't actually busy with working from home or something, then another thing I like to do is try to really embrace/enjoy/appreciate the fact that I have a WHOLE DAY to myself to do whatever project I want... cook something fancy, organize a corner of the house that's been driving me nuts, etc. (yeah, I'm really exciting, LOL)
but ultimately, I try to avoid having too many of those days because they do make me really sad. do you work from home or is there something else that forces them on you?