When I was a kid in New York in the 1940's and 1950's it was a custom to fill a sox with flour and wack the behinds of other kids, as well a chalking up their clothes. As you would strike you would yell "Hawoleen".
Parents let their kids go to school in old clothes on the holiday.
Is it still a NYC custom??
The Village Voice is a free newspaper you can find on many street corners around the New York City. It comes out weekly and there are interesting articles, events postings, venue listing ect...
I often pick one up to see whats going on around the city, It's a mine of information ;-)
If your new to town and not sure what to do pick up a copy of the village voice or the new york press. Both these are weekly rags coming out usually early to mid week. Both, especially the voice, are like the bible to any sort of cultural event in the city. whether it be music of any kind, theatre, movies, etc. both are online as well.