From the Wikipedia public announcement:
"Today, the Wikipedia community announced its decision
to black out the English-language Wikipedia for 24 hours, worldwide,
beginning at 05:00 UTC on Wednesday, January 18 (you can read the statement from the Wikimedia Foundation here). The blackout is a protest against proposed legislation in the United States — the Stop Online Piracy Act (SOPA) in the U.S. House of Representatives, and the PROTECT IP Act (PIPA) in the U.S. Senate — that, if passed, would seriously damage the free and open Internet, including Wikipedia... "