I have mountain lion and I am using mail. One time when I had mail open a sidebar appeared on the right that had some of my calendar events in it. Well it disappeared, but I would like it back. Any clues?
I don't believe such a thing is directly linked to Mail (unless perhaps you respond to an event invitation) but I think what you're talking about is the Notifications bar. Click the symbol at the very top right of your screen (it looks like three horizontal lines) to show it.