Ergot, a fungus that typically grows on rye, had been used for centuries as part of natal care, both to induce labor and to mitigate bleeding, though nobody understood how. Eventually, chemists in the United States identified lysergic acid as the active ingredient. Albert Hofmann was working at Sandoz Laboratories in Basel, Switzerland, in the 1930s, and was tasked with trying to make pharmaceutically viable compounds using lysergic acid. On November 16, 1938, Hofmann synthesized for the first time lysergic acid diethylamide (LSD) and fund it had no pharmacological effects, so he set it aside. For five years.
In April 1943, Hofmann decided to take his baby out for another test run, and accidentally ingested some, though he had no idea how--he originally assumed that what happened next was the result of inhaling a chloroform-like solvent in the lab. In any event, he was soon "tripping balls," as the kids day.
"Immediately, I recognized it as the same experience I had had as a child," he said years later. "I didn't know what caused it, but I knew that it was important."
On April 19, Hofmann took history first intentionally LSD trip, ingesting the tiniest amount, the effects of which came on as he was riding his bike home from the lab. This is now know to LSD enthusiasts as Bicycle Day. Hofmann would drop acid dozens more times, but eventually gave it up, saying "I know LSD; I don't need to take it anymore."
Hofmann's discovery will go down in history as one of the happiest of accidents.