Blue T-Days are the two days in the week that start with "T" as in Tuesday and Thursday, those are the days you wear blue underwear or panties.
Girl:"What is today???"
Guy:"Hold on let me check." *glances down at their underwear real quick* "Tuesday or Thursday."
Girl:"Blue T-Days?"
Guy:"Yep."
Girl:"Oh."
Hey, did you know December 14 is National Roblox T-shirt Day.