Click into Venue Settings > Hours of Availability.
The hours of availability are your venue's broadest opening times. Times outside of these hours will be displayed in a darker shade of gray on your scheduler. Regular users will not be able to book outside of these times. Admins can create bookings at any time.
Different availability times per space
If bookable spaces have different opening times you can create deny booking rules in the spaces settings area. For example, perhaps the bookable times for the venue are 9am - 6pm, but one space is only bookable from 9am - 3pm. In this case, set your venue hours of availability to be 9am - 6pm, and use the booking condition template "Deny all bookings" to deny bookings from 3pm - 6pm for that one space.
One-time changes to space availability (e.g. public holidays, maintenance windows)
Admins can create unavailable type bookings from the booking window to block out calendar dates and times. This might be for public holidays, one-off closures or maintenance times. Use the repeat / recurring feature to block out several days in a row or a pattern of calendar days. Review the article 'Creating Bookings as an Admin' for more information on the three booking types.