What kind of festivals are you looking for?
Many are advertised in the Penny saver which can be found on line at
www.nypennysaver.com Another good source would be the Journal News.
The biggest reason why its hard to find these events is that there is not one department that organizes them. Yorktown, for instance, has a Rec Department as does Westchester County and Croton on Hudson as well, but many of the events are done by private or non government entities.
Here is a link to Tourism and Recreation in Westchester. You can also find info for various towns on my site too.
Tourism & Recreation (http://www.stlawrenceproperties.net/tourism-recreation-westchester.php - broken link)
If you are looking for something specific, I was born and raised in Yorktown, so I know a lot that is not published.