Buy them in advance if you want a specific day or time.
You can buy the tickets right at the gift shop the same day, but sometimes they have their tours booked until late in the afternoon ? You might want to purchase them online .
In advance or right at the door but you will have to wait for the next tour. They are every half hour and last 1 to 2 hrs depending on which tour you choose.
I was able to buy same day tickets on Monday. Had to wait about a half an hour for the next tour to start.
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