Also, just leave registration open all the time. Nice simple "Register for next season" button. On the registration page "Next season starts Feb 1st, 2017. You will receive an email 2 weeks earlier as a reminder (and to verify you're still interested."
Also, back in early blitz days, the Farm league was actulay pretty popular, but I was just too busy/lazy to keep it going. So a "join at any time C league" could be a good idea, which I think someone brought up above already.