According to Jeff Atwood, today's the day:
When you're hired at Google, you only have to do the job you were hired for 80% of the time. The other 20% of the time, you can work on whatever you like – provided it advances Google in some way. At least, that's the theory.
Although the concept predates Google, they've done more to validate it as an actual strategy and popularize it in tech circles than anyone else. Oddly enough, I can't find any mention of the 20% time benefit listed on the current Google jobs page, but it's an integral part of Google's culture. And for good reason: notable 20 percent projects include GMail, Google News, Google Talk, and AdSense. According to ex-employee Marissa Meyer, as many as half of Google's products originated from that 20% time.
He goes on to ask if your company is ready, and offer some suggestions how to implement it. I think it's easier to do when you don't have billable-hour pressure, but still, we at my company do manage to get some goofing-off time in.
Or, put another way, "why is this day different from all other days?"