Hello.
1) The ability to define as unavailable specific dates based on season filters, possible in the Booking Calendar Business Medium or higher versions. Check more about this feature here: https://wpbookingcalendar.com/overview/#availability
Watch it in this video guide https://wpbookingcalendar.com/help/booking-calendar-video-season-availability/
2) In the Booking Calendar Free version the only way to make some dates as unavailable, its to make the blank bookings for such dates at the Booking > Add booking page.
Kind Regards.
