We went on a whim. The line for tickets was short as was the wait for our tour to start. Highly recommend a restroom pit stop before the tour.
You can buy tickets anytime
You can purchase the tickets same day
When we bought on Groupon, the vouchers were not for any time slot. We just showed up and they let us know time time openings they had. We only waited 10 minutes on a Sunday afternoon for the next tour.
When i had gone i purchased a ticket for the tour that started a few minutes after i arrived. It fully depends on how busy they are as they do cap tour groups at 26 people
It seems like you need more time in advance if you buy online than if you buy in person. When I looked online at around 9:30am they only had tickets available for the next day, but I was able to purchase tickets for the same day (45 min later) by going there in person.
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