420
From Wiki Weed
420, 4:20, or 4/20 (pronounced four-twenty) is a code-term that refers to the consumption of Cannabis and by extension, as a way to identify oneself with cannabis subculture or simply cannabis itself. Observances based on the number 420 include smoking cannabis around the time 4:20 p.m. (with some sources also indicating 4:20 a.m) on any given day, as well as smoking and celebrating cannabis on the date April 20 (4/20 in U.S. form).
