4/20 Day goes back to 1971 when a group of high school students called the Waldos began a journey to search for an abandoned cannabis crop, using the grower's "treasure map." They would gather at 4:20 PM to search for the missing pot, using the phrase "4:20" to signify that they would be meeting.
The story about "4:20" spread among stoners and 4:20 PM eventually became the socially acceptable time of day for people to light up. Eventually, 4/20 Day became a thing, and users began consuming cannabis products at 4:20 PM on 04/20 every year.
I didn't light up yesterday, but several of the XM Sirius radio personalities that I listened to while driving yesterday were looking forward to lighting up at the designated hour.
One last thing for my current and future employers: I don't smoke pot.