There was a puzzle throughout the xkcd book we published over at breadpig, which some ambitious xkcd readers managed to crack after months of work.
The solution was a set of coordinates, time, and date with lots of love. That day was this past Saturday the 26th. We had a great time at Golden Gate Park (as evidenced by these photos). There was a bobcat in a box, an xkcd kite (no camera, unfortunately), delicious xkcd cupcakes, and lots of glorious geekery!