Appointment Lead Times control how far in advance a client can book an appointment. The default minimum lead time is 8 hours and the default maximum is 365 days, but you can easily change these.
In the Settings section (gear icon at the top right), click on Appointment Lead Times.
You'll have the option of making the minimum lead time anywhere from "none" (0 minutes) to 5 days.
The maximum can be anywhere from one day to one year.
Be sure to save your changes when you're done!
Tip: Avoid making your Appointment Lead Times too restrictive, as that can cause online availability problems. Set lead times n such a way that your clients can always find future availability. For example, it's not a good idea to have a minimum lead time of 1 day and a maximum lead time of 2 days, because it's too limiting.