The company, founded by Larry Page and Sergey Brin in 1998, traditionally marks its birthday on the Google homepage on September 27.
In 2004, its 6th birthday Doodle went online on September 7 and in the year before that, it was September 8.

In fact, none of these dates seem to have any particular relevance. The company's own history lists its incorporation date in 1998 - the nearest thing to a corporate birthday - as September 4.

Details and cakes at: http://www.obiogo.com/search-engine-giant-google-celebrates-her-18th-birthday-today-sept-27th/