You can create a pass that customers can purchase today, but isn't active for attendance until some date in the future. For example - in December you start selling a 'New Year, New You' pass. You can set the active (start) date on the pass to be January 1st.
If a customer purchases that pass, and then comes to class in December, that pass will NOT be used for class attendance. On Jan 1st it will become active and eligible to be used for attendance. You don't have to do anything to 'enable' the pass - it happens automagically!
To change the start date....
Head into Manage Passes, then click on the Pass type you would like to edit
Click 'Edit The Pass Definition' button
Change the dropdown for when the pass becomes active for attendance to on a specific date, and enter the date.
If you change an existing pass, the new start date will only impact any future passes assigned to customers - it will not affect passes that have already been assigned.
This table gives you a good overview of your options regarding pass start dates: