I was on a mortar range at Camp Pendleton, California.
Specifically, I was sitting in front of the tent (two shelter-halves "pup tent") I shared with my squad leader, Corporal Hoth, eating an MRE for noon chow.
I already had reason to remember that day. That morning my platoon (81mm Mortars, Weapons Company, 3rd Battalion, 24th Marines) had arrived at the range. I was a few feet away from our commanding officer when a 155mm artillery round burst overhead. Short round from some cannon-cockers, miles behind us and firing into the same impact area. Everyone jumped about four feet in the air and then of course the CO put on one of those "happens all the time, but I'm cool under fire" body poses.
So anyway, there I am chowing down on "ham slice, 1 ea." or something and a few feet away another enlisted man who had grabbed a recent news magazine or something to while away any field leisure time and was just now browsing through it, said it.
"Ow, wow. Robert Heinlein died."
That was June, 1988. Heinlein had actually died on May 8, but this was back in the days before anyone who cared about anything got instant notification about that thing via the Intertubes. You had to actually look, and usually pretty hard, for the information you wanted unless it was information on the order of "Pearl Harbor Attacked!" or "JFK Assassinated!"
So anyway, the reason I'm remembering that I remember where I was when I heard Heinlein had died is because that very phenomenon is mentioned in the introduction to Learning Curve, the first volume of William H. Patterson's two-volume Heinlein biography. It covers the period from Heinlein's birth in 1907 to the effective midpoint of his life, 1948.
I'm racing through the book now and it's a fascinating read. Normally you'd see a "buy it" link with a cover photo in the right sidebar, but apparently Powell's doesn't have it in stock at the moment, so here's Claire Wolfe's Amazon affiliate link.