Two Road to Damascus moments that come to mind right away.
1) I guess I must have been in or just out of sixth grade, and my brother, who was either a high school senior or a newly minted grad, was joining the Columbia House Record Club. He decided to go with cassettes--partly because, for some reason, my parents had bought a Craig portable stereo cassette player a couple of years earlier (the detachable right speaker was hinged, and it fastened to the base unit with a clasp), but possibly also because even then he had his eye on a Triumph TR6 that he hoped to buy with money from his road-construction job. The rosewood paneled dash had a built-in cassette player.
He was having trouble filling out his dozen selections for a penny (or whatever it was), and scanning the ad, I liked the graphics of Roger Dean's Yes logo on the cover of Close to the Edge--I knew nothing about the band, though--and persuaded him to add it to his cart. When the shipment came, I think he gave it a quick listen and was sort of nonplussed by it. Wasn't really his bag--or mine,, necessarily. (I was into Chicago, Elton John, Jesus Christ Superstar, Grand Funk, and the late Beatles, and with three older brothers, I'd been exposed to everything from Simon & Garfunkel to Stevie Wonder to ? and the Mysterians to Freak Out. But not prog. From the opening synths under birds and running water, though, I was intrigued, and when the guitars-bass-and-drums hit hard, a minute in, I was floored. 18 minutes later I emerged from a trance, mind duly blown. I couldn't believe you were allowed to do something like that on a rock album!
2) I was out of college and bouncing around, not really knowing what to do next. I'd gotten close to a favorite prof--a brainy, complicated ex-Marine whose musical tastes ran from late Beethoven to Bob Seger. By this point, meanwhile, I was also a Yes-and-Genesis freak, but also deep into Steely Dan and Joni Mitchell and Peter Gabriel and Rickie Lee Jones and...I donno, The Roches, maybe? Anyway, we were hanging out in his den-slash-library when he told me, with a slight air of mystery and foreboding, that I had to hear an album a friend had lent him. He seemed a little spooked by it, and as he pulled it out of its sleeve he pronounced, in an oracular way (he wasn't given to this sort of thing), that he though this might be the "future of music." 20 seconds into "Sharkey's Day" (it was Laurie Anderson's Mister Heartbreak), my mouth was hanging open. I'm sure I sat stock-still, transfixed, through both sides of the LP.