Wikipedia main page says:

Imagine a World
Without Free Knowledge
For over a decade, we have spent millions of hours building the largest encyclopedia in human history. Right now, the U.S. Congress is considering legislation that could fatally damage the free and open Internet. For 24 hours, to raise awareness, we are blacking out Wikipedia. Learn more.
Contact your representatives.

(Tastefully done)

Apparently they decided on a 24-hour blackout, rather than the 12 hr reported earlier.

John