Snape's Date of Death
and Fred's and Voldemort's and Lupin's and Tonks's as well.

Rowling mentions the date of the final battle early on in this video clip (which coincides with Victiore's birthday a year later). I wasn't quite sure exactly on what day the final battle took place, but now that I do, I can throw an honest to goodness Death-day party for Snape!

Also, how cool is it to know the middle names of all the children in the Epilogue and beyond!