Bicycle Day celebrates Swiss scientist Albert Hofmann’s discovery of lysergic acid diethylamide or LSD in 1943, and the subsequent, and at times harrowing, ride home on his bicycle after deliberately ingesting a dose of the drug.
Three days earlier, he had absorbed a small amount of the drug either through his fingertips or by accidentally ingesting it.
On Bicycle Day, he took 250 µg of LSD intentionally and, fearing he had made himself ill, cycled home from his lab. During his bicycle ride, he experienced the effects of LSD, making this the date of the first ever acid trip.
He wrote about his experiments and experience on April 22, which was later put into his book LSD: My Problem Child.
This day (for LSD users) is when they celebrate the discovery and first ever use of the drug.