The tree will be up until January 7th. Make sure to check the times it is lit so you can see the lights. You should find the details here https://www.rockefellercenter.com/blog/2018/11/08/2018-rockefeller-center-christmas-tree/. Enjoy it!
According to their website The Tree will remain lit and can be viewed until 9pm on January 7, 2019.
Sorry you will miss it... Tree usually goes down right after new years
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