The number four hundred and twenty is, for some reason, intrinsically associated with marijuana enthusiasts, who use the number (pronounced "four-twenty" by them) to signify that they smoke weed, and celebrate April twentieth (04/20) every year as a sort of holiday.

Presumed origins of this "code" date back to 1971 where a group of students from San Rafael, California had devised a plan to steal some cannabis crops that had been abandoned, the plan was referred to as "4:20 Louis", this was due to their meeting location being the Statue of Louis Pasteur found in the San Rafael High School, and their meeting time at 4:20PM. Since then, 4:20 has been passed out as a code word of sorts by the marijuana smokers to refer to as the time to smoke weed.

On e621, the 420 tag refers to images or animations depicting characters smoking marijuana in some fashion, whether through hand-rolled cigarettes (joints), or smoked in a device such as a pipe, bong, or hookah.