If you have entered your birthday on the Google Profile, Google will show a Birthday Doodle on the Google page. Read more from Google itself
A birthday surprise
Because doodles are such a fun part of the search experience, we thought we’d share a fun little way Google will help celebrate your birthday. When you include your date of birth on your Google profile, you may notice a special treat on the Google homepage on your birthday (be sure to sign in). Click on the doodle for another birthday surprise!
Read more here
You may try this by changing the birthday on the Google Profile