Clean it now. That is, if you haven’t sent to your list in a long time, offer them the chance to re-opt-in. Then you can confidently send messages in December and January knowing you are reaching people who care.
Create sign-up tools now. An iPad linked to your account or a printed paper pad will make it easy for people to sign-up and then you can invite them back with special offers in January, February and March when your sales may be slower.