Apple's annual developer conference in San Francisco.
Apple Giving $25 App Store Gift Certificates to WWDC Attendees
The gift card is in celebration of "25 years and coding", as noted by @HarrisonW1998:
Apple is offering a live video stream of the Keynote on Monday. We'll have live text and photo updates here on MacRumors, as well as live Twitter updates at MacRumorsLive.