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When people take selfies of themselves on Sunday. Selfie Sunday was made to have a purpose of selfies on a particular day which in this case is Sunday.
Girl- "Why are you taking a selfie?"

Other girl- cause it's selfie Sunday!
by Brownie743 November 11, 2013
A picture taken of yourself by yourself and posted on a social media site on any particular Sunday.
"Do not photobomb me, this going up as my selfie sunday."
by instagrammarnazi February 23, 2014