The Monday after No Shave November, where you shave all your facial hair except the mustache.
-Dude, are you excited for Mustache Monday?
-Yeah, mine's going to be real nasty
The first Monday after the end of No Shave November.
On this day, men shave their faces after a month of neglect; leaving nothing except a mustache style of their choice.
Everyone thought John's fu manchu was hilarious on Mustache Monday.