I would recommend you to get zim for that, xournal might do it as well. I merely write diaries, although if there's are something important I just create a calendar event.

Zim is unbelievably useful application, at first I thought I would never use it but once I started using it its addictive. It may have more functions than what you need but I am sure not less than that. Try it its a desktop wiki application, you can write date based articles/diary as well as categorized articles.