As far as I know there is nothing called, "State Historic Park" in Downtown LA. I think they are just saying that it's going to be held in a state historic in Downtown LA.
The only park I am aware of in Downtown LA is Pershing Square which is kind of small but if they closed the streets off it might work.
Maybe they mean a park near downtown, maybe they will do it in Griffith Park or...
Maybe they will do it in Barnsdall Park, that's where they had the Arthur Fest...
http://www.barnsdall.org/
Or somewhere else, who knows?