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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
by Nashty November 27, 2008
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.
by Loudly December 17, 2009