I don't see any slots available for reservation during the week. Does that mean the site is closed, or fully booked?

Certain time slots for trips on the Grand Elephant, tours of the Galerie des Machines, and the Marine Worlds Carrousel, are open to reservation on weekends, French school vacations, and French public holidays.

If no availability appears, this means the slots are either full or unavailable. In this case, you can buy tickets on site for that very same day.