It really depends on what you want to see. In the winter there's always the Macy's parade that happens in November and then the ice skating or annual tree lighting at Rockefeller Center.
The summer it might be best to do those outdoor activities, like Coney Island, Statue of Liberty, or even Niagara Falls.
New York has something going on all year round! It just depends what kind of weather you want to be in. The winters in New York are very cold so if you're not to keen on that idea go in the summer.
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Best Time to Visit NYC