Fall of '78. I had just started college. I came across a flyer advertising "Bilbo's Birthday Party" - the annual recruiting bash for the university's Tolkien society. And already being a big Tolkien fan, I of course went. And there were a lot of D&D players, wargamers, SCA members, SF fans etc. there.
Also, I encountered my first game store there, just off campus. I had seen ads for Metagaming's Microgames before, and I was intrigued by "Melee" and "Wizard." But I wasn't willing to do mail order.
In the store, though, I saw those microgames "on the shelf," along with dice, minies, old issues of Dragon... and the 1st ed PHB and MM (Not the DMG, though - that hadn't come out, yet).