Writing a diary entry is essential especiallyon bad days. It really helps. When you have a bad day, it could be because of so many small reasons. It's rarely a big incident. But mostly it is the accumulation of small bad incidents. And then there is the concept of overgeneralization which means taking one bad incident and writing off the whole day as bad day. It could be different for different people. But what ever you are feeling at the moment. Write it down. Without judging yourself. No matter how trivial thought it is that has been eating you all day, write it. Be honest. If you made a mistake then write it too. If you think someone you know made a mistake but you could not say it at the time, write down that as well. Get things off the chest.