Send mail at end of subscription
To warn people they should come see a rezoman before their subscription reach the end, an email could be sent some time before it happens.
The exact time still has to be determined but a good choice would be one mail a few days before the end so the user can see someone even if there is a weekend or whatever.
Additionally, another mail could be sent earlier (like a month before) too. ( Of course, when you are paying monthly, there should not be an email to tell you have 1 month left ).
Maybe think about letting each association decide the timing and the number of warning mails.