In Settings / Configuration / Advanced / Client Booking Limits you can set a value in weeks or months.

• Weeks means that on any given day, your clients can book ‘X’ weeks in advance.  In other words, every day allows bookings ‘X’ weeks in advance.
• Months means that the future booking date only becomes available on the 1st of a month, ‘X’ months in advance.  For example:  If you chose only 1 month in advance, by the 28th of the month, clients could only book 2 days in advance.  Hence, this option is limited to 2 months or more in advance.