You can decide how many weeks or months ahead of time clients can book appointments. If you choose 1 week, clients only see openings in the next 7 days. If you choose 1 year, clients can pick a day 12 months in the future.
To update this, use the "How far in the future may clients book appointments?" setting under Setup > Booking Rules.
You can also decide how much advance notice to require for appointments scheduled online.
If you don't mind last-minute appointments, you can let clients book openings just an hour or two from now. This can be a great way to fill gaps in your schedule that would otherwise be unpaid downtime. You may want to turn on text message notifications to your cell phone in case someone books on short notice - you don't want to be surprised!
If you aren't onsite or like to plan your day ahead of time, you can bar clients from scheduling appointments the day of or the day before. In fact, you can require up to a week's advance notice for online booking. That's a good option if you don't get online or check email frequently, or if your calendar fills up quickly.
If you select '1 day before', clients will be able to schedule online until midnight the day before, after which no openings will show. Whereas '24 hours' provides a full one-day window, so that as each hour passes, openings 24 hours later will cease to be presented.
To update this, use the "How far in advance must clients book appointments?" under Setup > Booking Rules.