Labeling online albums to be in alpha order?

I have my photo albums for a group of photographers to post in - in order labeled "week1", "week 2", etc. Everything was fine until I got to "week 10". It is now after week 1, instead of after week 9. What else can I use besides numbers to keep them in alpha but also chronological order? If I use letters, that will only last 26 more weeks, and I would like to go longer than that.