Google knows special days

Today when I logged to my Google account i also noticed the special Google logo change!

http://searchengineland.com/a-google-logo-for-your-birthday-53089

http://www.labnol.org/internet/google-birthday-logo/20337/

http://www.telegraph.co.uk/technology/advice/9949627/How-does-Google-know-my-birthday.html