On Fridays from 6.45 to 7.15 something like that, it is free but I do not recommend it, because of the super queues, and the noise. If you want to visit it, go any other day, which is pleasant to visit.
On Thursdays if I remember correctly from 16:00 to 20:00 it is free etrads. Take a good look at it on the web, it puts it.
You have to pay, you can take the New York pass and it will be cheaper
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