January 22nd 2013, according to an official tweet Google announced its first algorithm update of 2013. It is sequel of Panda update, an algorithm launched in February 2011 ( http://goo.gl/Og8sF ). This current episode is 24th refresh to the Panda update.
Panda #24 is supposed to affect 1.2% of search queries. Many websites have already got this effect and others might be getting in next few days.