What kind of anstrengend?
Yes, you can say a tough day, if you mean it was difficult, hard to endure, and it didn't end happily. A tough day is basically a bad day for you, one you don't like to think about. Maybe you would even rather it had not happened, wish you hadn't had to go through it. Not always, but often, 'tough' suggests some emotional difficulty, something that made you feel sad or discouraged.
A day can also be busy, long, tiring, stressful, etc. without necessarily being as bad as tough, especially if at the end you feel it was worth the effort or there was a good result in the end. It's not an absolute distinction, but to me there are other things that are more common if your meaning is a little less negative.