Slap Your Older Twin Day
National Slap Your Older Twin Day, occuring annually on April 17, gives the younger twin a free pass to slap their older twin. Maybe it's pent up resentment for being beat out of the womb, maybe it's just for fun, either way: they're free to slap.
"My twin slapped my today, but it's okay because it's April 17, so it's national Slap Your Older Twin Day, and she's the younger twin"