Forty years ago Monday the Grateful Dead played in upstate New York at Cornell University’s Barton Hall. Memorialized as “5-8-77” on a million cassette tapes, it is considered their best, a legendary show, the band’s Holy Grail. A good friend ran the concert commission and assigned me, a lowly freshman, to turn on the house lights when the show ended. Things have changed over the past 40 years, but it took time to make out what was illuminated that night.

The Grateful Dead began playing in 1965 as the Warlocks, right before...