Even though Halloween type celebrations have been around evere since immigrants started arriving in NYC it didn’t become a widely celebrated event in NYC until around 1939/40.
By that time New Yorker’s knowledge of the pagan harvest festivals and Christian rememberance of the dead rituals of previous times had been absorbed into a range of usual Halloween activities such as wearing scary costumes, trick or treating, jack-o-lantern carved pumpkins, horror films and bonfires.
If you come to NYC during the celebration of Halloween these days you will see hoards of well chaperoned adorable young New Yorkers in costume going trick or treating in their neigborhoods. Otherwise know that the big, big Halloween event is the Village Halloween Parade which you can take part in provided you know all the rules and you’re wearing a costume. See
Village Halloween Parade 2015
As well, given that Broadway/acting/media is very big in the city there is a huge range of specially created ‘Haunted Houses’ that you can visit to be duly scared out of you rind. For a delicious behind the scenes peek see the video. And see this article on the best of them in NY Haunted House Guide
There are also lots of Halloween themed events and many of them are advertised on the Village Halloween Parade website. And Halloween is still a Christian festival and most churches in NYC will have “All Hallows Eve” services which you can attend.
One of the most fun things you can do as well as attend the parade, is simply ride the subway. You will see any number of wonderfully costumed New Yorkers either going to the Parade or just taking advantage of this being a time of year when everyone gets a free pass to dress up.