If you call Saturday the first day of my summer vacation (which you should not because it was a Saturday), then I spent it at Jack's 4th birthday party (where he gleefully defeated a burglar!), planning the Asia trip with Jason, and staying up way too late writing a talk.
If you call Monday the first day of my summer vacation (which you should not because it was also Memorial Day), I spent it seeing X-Men: Days of Future Past with Lisa and her brother followed by lunch at Panera, making Miranda's awesome salad dressing (and salad) for a barbecue at Rachel's that evening, and more trip planning with Jason.
If you call today the first day of my summer vacation (which you should), I spent it taking Natasha to the vet for shots so she can get shaved tomorrow, joining Fara and Tammy at Fara's house for a delicious pot-luck lunch and a screening of The Bookthief, errand-running, dying my hair (yet again), and general trip preparation for New York City.
Any way you call it, it's been a good start to the summer.