May 17 is also, if I am not badly misremembering, Marriage Equality in Massachusetts Day. (And my brother and sister-in-law's wedding anniversary--it still pleases me, no matter how much my brother has relaxed politically in the last ten years, to have those two permanently linked on the calendar. *g*)

But it could surely also be If You're Not Dead of Smallpox, Thank a Vaccination day. :)