In the world of cannabis, the time 4:20 is slang for smoking marijuana (e.g. pot).
For those of you movie trivia buffs, there is a myth that all the clocks in the movie Pulp Fiction are set to 4:20. While the clocks on the wall in the pawn shop are indeed set at that time, there are at least two scenes where the clock is set at 7:22 and 8:15 respectively.
For those of you cannabis buffs, if you want an interesting summary of how 4:20 became slang for smoking pot, you can check out HuffPost for the scoop!