There's about a million and a half street fairs to choose from this weekend - many put on by local civic associations and schools. A full list for this Thursday-Sunday can be found on the About.com blog of Pamela Skillings at http://manhattan.about.com/od/eventsandattractions/a/streetfairsmay_2.htm.
The busiest is likely to be the Times Square Fair on Sunday (2 guesses where you'll find that one), and the Salute to Israel Parade (I always just called it Israel Day - didn't realize it had a name). So, officially we are also into parade season. Yeah, I guess they count as festivals, too. Salute to Israel is going up Fifth Ave from 57th Street to 79th on Sunday starting at 11AM and lasting till 4PM. Just look for all the blue and white! For more information visit: http://salutetoisrael.com/parade/index.php?option=com_content&view=article&id=54&Itemid=71