why do I keep forgetting tomorrow is a holiday?

For maybe the fourth time today, I just said something to a co-worker about working on stuff tomorrow (though luckily it hasn't been the same co-worker each time).

I do that on Fridays too, I'll be like "OK, we'll finish that tomorrow" and they go "uh, I'm not coming in on Saturday."

Oh well, I'll remember it tomorrow morning. I hope.