Panda was Google's first named quality update of the modern era. Google's post of 24th Feb 2011 called it "a pretty big algorithmic improvement to our ranking", launched "in the last day or so".
Our history dates the launch to 23rd Feb 2011 on that basis. Google gave no exact day.
The scale was unusual for Google to state: "a change that noticeably impacts 11.8% of our queries".
Its purpose, in Google's words, was "to reduce rankings for low-quality sites". Google described those as "sites which are low-value add for users, copy content from other websites or sites that are just not very useful".
The same post promised "better rankings for high-quality sites", meaning "sites with original content and information such as research, in-depth reports, thoughtful analysis and so on".
The launch was US-only. On 11th Apr 2011 Google rolled the change out "globally to all English-language Google users" and added user feedback signals, affecting "about 2% of U.S. queries" on top of the original.
Google never used the name Panda in those posts. Its current ranking systems guide does, listing Panda under retired systems: "it evolved and became part of our core ranking systems in 2015".
