It's going to be a sleepover at Facebook the night before the big IPO.

This is how Facebook describes a "hackathon": " Hackathons are a big tradition at Facebook. They serve as the foundation for some great (and not so great) ideas. It gives our employees the opportunity to try out new ideas and collaborate with other people in a fun environment."

Basically the company's employees spend the night working on their ideas to see where they go.  Everyone at the company is invited.

This is something they have done in the past, but this will be their last such endeavor as a private company.

The next morning Mark Zuckerberg will ring a bell that will make many of these employees millionaires and billionaires.