While not a PR, this the first race I've done with someone else, just running at their pace and egging them along. My friend did an awesome job on her first 10K, and I had a great time. As fun as running is for me, it's so great to watch someone else getting joy out of it.
(thanks to my brother-in-law for this one!)
And really, is there a better way to spend your birthday than raising your heart rate a bit?