A special occasion recognized by the Google homepage. Google traditionally commemorates the event with some kind of doodle. Sometimes its an actual holiday like Valentines day, Chinese New Year, or Independence day for another country. Sometimes its an obscure reference to someone who died over a century ago.
Man1: Dude, did you know that May 31, 2013 was Julius Richards Petri's 161st birthday?
Man2: I'm sorry. I don't celebrate Google Holidays.