You are scheduled for an FP appointment by date and time. If you are unable to make it, most DOs allow walk ins. Wednesdays and Saturdays seem to be preference for walk ins.
Do all offices open Saturdays? Is there a way to confirm? phone number or something...
They are called ASC (Application Support Centers):
https://egov.uscis.gov/crisgwi/go?action=offices.type&OfficeLocator.office_type=ASC
my office says open to public Mon-Fri and military person on Sat. Does that mean not open to public on Saturdays?