Sure, what you'd want to do is set up a system event to display a message at the time of check-in.
Here are the steps:
<FirstName> <LastName>'s birthday is this month! (<Birthdate>.)
Test a check-in for a patient whose birthday is this month.
If you want it to be more specific to if they have their birthday that day, I can do instructions for that as well.
I can also give you a call to assist with this setup if you'd like.