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2.
1. Third Sunday in June, co-celebrated with but not to be confused with Father's Day, reserved for sperm donors, sometimes also called Baby Daddys, who fail to recognize or acknowledge their offspring;

2. Third Sunday in June, co-celebrated with but not to be confused with Father's Day, reserved for deadbeat dads, sometimes also called Baby Daddys;

3. Third Sunday in June, co-celebrated with but not to be confused with Father's Day, reserved for men with children after an acrimonious divorce.
"Mommy, you know how you call the sperm donor a motherfucker? Can we get him a gift?" "Gift for what son, Motherfucker's Day?"

"Mommy should we get daddy a gift?" "Don't call him Daddy. You ain't seen him in 3 years. You 8 years old and that motherfucker is 7 years behind in his child support? For what, Motherfucker's Day?"

"Mommy Sunday is Father's Day. Dad wants me and Mary to eat dinner at his house." "Fuck Father's Day! Shit it's Motherfucker's Day to me!
by Won-J'az Edued May 15, 2013
 
1.
The day after Mother's Day, dedicated to all motherfuckers everywhere.
I gave my next door neighbor flowers for Motherfucker's Day. I told him "you may not be a woman but you sure are a mother".
by Quaoar May 09, 2006
 
3.
short for "Father's Day"
John: Hey Doug, Happy Motherfuckers Day, motherfucker !

Doug: That's today? Shit, I forgot to buy my dad a present.
by babybeeks July 22, 2014