Google rolled out a Panda update late on Friday. Google announced the update on twitter saying:

Looking into the wording on this tweet, we can learn a few things:

(1) 1.6% of the search queries were effected, which is small but large for those hit.

(2) This is specifically a “refresh” and thus it seems like no new signals or algorithm changes were made to this Panda update. In short, they simply reran the algorithm on their index.

There was speculation that this update was being rolled out earlier but I am not sure if it was specific Panda related or not.

The previous Panda update was 3.3 taking place on February 26th or so. Here is the timeline:

Past Panda Updates:

Panda 3.4 on March 23rd
Panda 3.3 on about February 26th
Panda 3.2 on about January 15thPanda 3.1 on November 18th
Panda 2.5.3 on October 19/20th
Panda 2.5.2 on October 13th
Panda 2.5.1 on October 9th
Panda 2.5 on September 28th
Panda 2.4 in August
Panda 2.3 on around July 22nd.
Panda 2.2 on June 18th or so.
Panda 2.1 on May 9th or so.
Panda 2.0 on April 11th or so.
Panda 1.0 on February 24th

There is a lot of discussion around this announcement.

Forum discussion at WebmasterWorld and Google Webmaster Help.