420
'420' refers to two critical times of day and year for marijuana aficianados, typically referred to as 'stoners.' It marks the time and date to get stoned.
4:20 PM
The time 4:20 PM, supposedly the time of day that Jerry Garcia died. is the optimum time of day to get high for stoners. It is exactly half-an-hour after the last daytime class, and provides enough time for stoners to gather in designated rooms and locations to light up a joint, bowl, or bong. On rare occassions, an observer can witness a phenomenon known as 'hot-boxing,' where a large amount of cannabis smoke fills a room full of stoners.
April 20th
April 20th is the international stoner holiday. While most people might not notice it, a significant minority may be missing from classes or work on this Day of days, presumably to smoke bigass bowls all day long. The day is also a significantly active day for the otherwise sedate NORML, who traditionally pass out hemp ribbons and pamphlets regarding marijuana legalization. NORML also sells brownies, though at $1 apiece, they are merely symbolic of magic brownies.
Date-Time Convergence Theory
If both of these events are cause for smoking copious amounts of pot, then the intersection of both the date and time is a truly legendary event. Every 20th of April, at 4:20 PM, a massive congregation of stoners leave the safety of their rooms to gather at a hidden location, and smoke up in an enormous circle of communal pieces and the dankest shit.