Ah, good point, Chris.
The standard advanced tickets are 'open' - so consider changing the day if you have the option.
Here are the weasel words if anyone hasn't seen them:
Thank you for booking your tickets with us,
We have been made aware that RMT strike action is set to cause significant disruption to West Midlands Railway services every Saturday through November and December. If you are due to visit us on one of these dates, please plan your journey carefully and consider alternative modes of transport.
As a result of this industrial action all trains in operation will be extremely busy given that we are now in the midst of the busy Christmas shopping period and the German Market in Birmingham city centre.
For the latest details on affected services, please visit the West Midlands Railway website. Dates could change at the last minute so we recommend you double check journey details just prior to travel.​