There are two settings, both found in your activity settings.
Opening of booking window
This sets how far in advance members can start booking a session. For example, if you set this to 15 days, a session becomes bookable 15 days before it starts.
When the booking window closes
This sets how close to the session start time bookings are cut off. For example, if you set this to 6 hours, members can no longer book once the session is 6 hours away.
Tag-based rules
For both settings, you can create up to 3 custom rules based on member tags. This lets you give different access windows to different groups — for example, allowing VIP members to book earlier than regular members.