As far as my addled brain can recall, it was sometime in the late 60's at a BriSCA stock car race at Brafield Stadium.
I vaguely remember soggy hot dogs and cold coffee - and a face full of crud from the track surface.
Another thing, a few have waxed lyrical about the evocative smell of Castrol R. Well for me, my particular 'special' smell that means I'm close to some serious kick-ass racing is that of 90% + Nitromethane. The stuff will sear the back of your throat and make your eyes stream if you're close enough, and the sound, well that's another thing.
Living close to Santa Pod in the late 70's / early 80's, many of my early racing memories are of drag racing there. Remind me to scan my old photos someday.