The social indoctrination to the bar scene that occurs on or immediately after one's twenty-first birthday. Typically, the guest of honor on a "21 run" is forced to consume copious amounts of alcohol in a short amount of time. This often results in illness, and, in extreme cases, loss of consciousness. The prototypical "21 run" occurs from 12:01 AM until 2:00 AM on the participant's birthday, this time being the first at which said individual is legally eligible to consume alcohol.
A man's friends who are already twenty-one pick him up and take him to the bars one minute after he turns 21, at 12:01 AM on a "21 run."