The event is the brainchild of last year's FoPAAS youth board liaisons from Paly and Gunn, respectively, Kristen deStefano and Joanna Tang, who are now off at college. Here's background from Palo Alto Online and background from FoPAAS on how the event came about.

The event takes places from 1-3pm at the Rinconada Bowl, across from the pool area. It's a perfect location, as the bowl provides a venue to promenade and socialize with other dog lovers, a stage for the judges, along with grassy areas surrounding the bowl to walk around with your pet.

As before, the focal point of the event is the dog costume contest, in which people can dress up their pets, and even wear a matching costume if they'd like, to be judged by a VIP panel. It's free to attend the event and enter the contest, just show up at the event and register to participate. Categories include:
Scariest Dressed
Best Family Theme
Best Couple Costume (dog and owner)
Best Dressed

Judges include Connie Urbanski, Acting Animal Services Superintendent at PAAS, Lydia Kou
Karen Holman, and Marilynn Goldberg.

This year's event also features food vendors Rick?s Rather Rich Ice Cream and the new gourmet hotdog vendor, Chez Franc. They've?ve booked a roaming dog photographer, and are holding a raffle, with prizes like a $549 robotic vacuum.

Special guest: Palo Alto Police Department K-9 Eddie and his handler Officer Brad Young.

Be there or proceed directly to the dog house!
See above link for photo credits.
Also: there is a Bark-for-Life dog costume event in Atherton earlier the same day in the morning, from 10am-1pm. So why not go to both? The Bark-for-Life event is at Holbrook-Palmer Park in Atherton, and benefits the American Cancer Society, see my blog post here about it.