The available days to book an appointment for a doctor depends on the doctor itself. Some choose to allow only 'same day' appointments, while others allow advance booking upto a week or more. Please check the calendar of the doctor that you want to consult with on the doctor's schedule page.
With a Sminq PLUS membership, you will always be able to book tokens at all doctors at least 12 to 18 hours earlier than non-members. This gives you convenient access to your doctor's slots according to your needs.