STD monday is the best day of the week in the army. On that day, you get to have sex with anyone you want (hint, STD). It's usually on a Monday and tons of babies are conceived with in that time frame.
Stacey: OMG, I had sex with Phil on STD Monday and now I'm pregnant!
Tara: You shoulda used protection. Now your gonna end up with an Army Brat.