This is something I read about a while ago and I think it is a tool that can be useful when we feel the world is dark and there is no hope of ever seeing the sun or feeling happiness again.
The idea is that through the year you write on slips of paper things that have happened that made you feel happy or grateful. They can be small things or life changing moments. These can also be written in a book where you can add pictures, or you could start a blog. Use whatever tool works for you but keep using it, making a note of happy things.
When you feel the darkness and SADness creeping in get out that jar, book or blog and give yourself the time to look through and remember those moments.
I strongly believe that one of the important tools in depression is the faith that one day you will feel better. Reminding yourself that you haven’t always felt this bad can help you know that you will feel better again some time in the future.