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2.
When you take Monday off work because you just need an extra day off. It's not Sunday and doesn't feel like Monday, it's Smunday. Similar to how Brunch is a hybrid between Breakfast and Lunch.
I was so tired after the weekend I called into work sick this morning; I'm taking a Smunday.
by kickback chick January 20, 2012
 
1.
Noun; The comination of the days Sunday and Monday. Often used to describe a 48 hour period of sleeplesness between Saturday and Tuesday.
"Dude, I havent sleep since yesterday, it's been one hell of a Smunday."
by KungFuCorey December 09, 2006
 
3.
figuratively spoken, the day between sunday and monday;

actually its a monday where you decide to not DO or GIVE a fuck, just like on sundays

the perfect smunday is a day, where you don't even know if it's sunday or monday
A: Are u going to school/college/work tomorrow?
B: Nah, it's smunday.

A: What day is it? Monday or Sunday? I didnt left the house today.
B: No fkin idea. Me neither.
by norbart, the pirate March 08, 2012