One month in advance, i.e. if you want seating on Dec. 12th, you'd want to call November 12th. The number is (714) 781-4400, opens at 8 a.m. Pacific time. It used to fill up fast, but that doesn't seem to be the case any more, so I doubt you have to worry about calling right at 8 a.m. People have called 2 weeks before and gotten seats. That said, I'd still try to call on the one-month day.
Cost is $59 for adults, $49 for kids (3-9) under 3 are free (if they're lap sitters).