Well im guessing you rang the number for a appointment so the operator should have checked the window you needed a booking.
They have either made a mistake or possibly they wanted to ensure no slots went unused so have the discretion to let you go one day early?
Ring them up and check.
When you go in person they check the paperwork quickly and the form before taking your money.
Good luck and let us know how you get on tomorrow with regards the phone call.