Hessler is pretty much the start of summer for me--it's such a hallmark in the year, as students are done with classes and graduation is always the same weekend, and (for the most part), the temperature tends to be pretty agreeable.
The street fair itself is usually incredibly congested, but that's how these types of events go. Just be prepared to be in contact with a mass of other bodies. Everyone is usually pretty polite and nice here. It's a hippie fest, so, you know, peace, man.
Definitely check out this event if you're into handmade items and solid people watching!