"Exactly"? You're kidding, right? This was Claremont, in the 60's; "when exactly" is a contradiction in terms. Especially having to do with the Midway. Anybody who tells you "exactly" when anything happened in Claremont before 1975 or so wasn't there; they looked it up.
The day, for me, was, oh late 60's into the mid 70's. Zappa had already been through, but the Padua Hills art colony was going strong.
If you can get one of the bumper stickers with your quote, its probably worth a fortune at the Pomona Art Museum. Fire was in the late 70's. I think the In'n Out might still be there, corner of Central & Foothill.