http://en.wikipedia.org/wiki/History_of_lysergic_acid_diethylamide
The events of the first LSD trip, now known as "Bicycle Day", after the bicycle ride home, proved to Hofmann that he had indeed made a significant discovery: a psychoactive substance with extraordinary potency, capable of causing significant shifts of consciousness in incredibly low doses. Hofmann foresaw the drug as a powerful psychiatric tool; because of its intense and introspective nature, he couldn't imagine anyone using it recreationally.[8] Bicycle Day is increasingly observed in psychedelic communities as a day to celebrate the discovery of LSD.
The celebration of Bicycle Day originated in DeKalb, IL., in 1985,[9] when Thomas B. Roberts, then a Professor at Northern Illinois University, invented the name "Bicycle Day" when he founded the first Bicycle Day celebration at his home. Several years later, he sent an announcement made by one of his students [10] to friends and Internet lists, thus propagating the idea and the celebration. His original intent was to commemorate Hofmann's original, accidental exposure on April 16, but that date fell midweek and was not a good time for the party, so he chose the 19th to honor Hofmann's first intentional exposure.[11]