I think numbering it by seasons is a good idea. And how about letting the new seasons always run 18 weeks as a remembrance that the contest only needed 18 weeks to get its own account. I still remember your post searching for ideas for a contest and then this great contest came to life - have you ever thought in the beginning that it's gonna get that far? :)
Anyway, I wish you continued success with the contest - this contest is for sure a part of things that makes steemit a nicer place.