I am in a bad mood right now. Even though it was a good day, I am right now still in a bad mood. And that's fine, sometimes you just want to be "there". This corona things and that everybody is just talking about it annoys me.
If you would always be happy you wouldn't even know how much it is worth to feel happy, the grass on the other side is always greener.
But one thing is even more important than understanding that. Read carefully; A bad day is fine, the crucial part is that a bad day doesn't lead to a bad week, and a bad week doesn't lead you to a bad month and a bad month doesn't need to mean a bad year.
It's what you make out of it.