Barry - here is the link to the Tulip Festival...they update the maps to let you know which fields are in bloom and when LOTS of events are. http://tulipfestival.org/
I know that the street fair is the 3rd weekend of April (weekend of the 20th)...but lots of times the tulips have been topped by then, it all depends on when the fields start blooming and how warm and sunny days we have!