A specific night, typically weekly, where a local bar offers an extremely large quantity of beer for a very economical price in a mug branded by the bar. For a nominal fee, you can purchase a mug, which you can use again week after week. Usually very cheap beer, and typically offered by college bars. Example: 32 oz. of PBR for $1 with mug.
Guy: dude, wanna go to mug night?
Other guy: I can't, I have a test tomorrow morning. Stupid registrar, only schedules this class for 8AM MWF.
Guy: That sucks!
When really good friends - preferably third years at their university (/college? <-- please this isn't for amateurs) - spend the night drinking from matching mugs. If it's done right, these friends are usually athletic and smokin hott.
Boy - Did you see those girls who had the mug night last Friday?
His friend - Hell yeah! I heard they're doing it again this weekend. We need to get in on that.
Girls from mug night - Ha. They wish, but unfortunately we're way outta their league.