Normal availabilities create time slots for days on a recurring basis but if you want to set availabilities that are available only for a specific date or range of dates you can use the "Schedulable availabilities" feature.

When adding or editing an availability if you check the "Applies to specific dates only" option the availability will only show for the date or range of dates you select:

Scheduled availabilities follow all the same rules as normal availabilities with the exception of showing up only on the scheduled dates, you can create and mix as many different combinations of availabilities as you'd like.