The days you don't feel like getting out of bed. The days everything in your project goes wrong. The days you think your idea is actually worthless and you're a fraud. The days you dread your investors asking for a status update. The days Murphy's Law is a bitch. The days you have a bug that just makes no sense. The days your client is being an ass and you have no desire to deal with them. The days your boss is in a bad mood. The days that one douchey coworker just keeps getting in your face.
The days that make you wish you were doing something else.
Oh yeah, I've had them - I wasn't being patronizing.
I just believe, due to the problem-solving behavior of engineers/techs/programmers, we tend to tunnel-vision instead of looking at the big picture.
If the big picture is going wrong, it's a problem. Now, a lot of bad days add up to a bad big picture, but if you're planning right, and you have a bit of luck, a few bad days will eventually get washed away, no matter the project.
I think Spolsky's description[1] of working at the bakery explains best how it works for me. My bad days don't seem to be anything catastrophic going wrong, just a multitude of tiny frustrations. Basically the opposite for the good days.
The days that make you wish you were doing something else.
I'm sure you've had "tough days"