How does Google know it's your birthday Today and how does it celebrate it?
Well the short answer is that you told Google when's your birthday, most probably by entering it in your Google+ profile.
This Google 'egg' was introduced back in October 2010, although I just discovered it now. Google actually offers instructions on how to set up your profile in order to get a Google Birthday Doodle. And yes, that's what you get for your birthday from Google: a Doodle with lots of cakes. It used to show confetti as well after clicking the Doodle, but now you only get redirected to your Google+ profile.
Google's Homepage showing my Birthday Doodle
I personally was expecting a few extra thousands of visitors on D3k from SERPs or at least a backlink from Google.com's homepage. For server stress testing purposes only, of course :). But I guess the Birthday Doodle will have to do for now :). Don't get me wrong, I appreciate it, I even printed a page (in high DPI) and seasoned it with some whip cream ... quite tasty!
In order to see your own Birthday Doodle you must go to a Google homepage. It can be a .com page or any local Google page, such as google.de, google.fr, etc. You obviously have to be logged in on your Google account in order to see your Doodle.
Besides seeing the Doodle on a desktop browser (any browser as long as you are logged into your Google account), you can see it on any iOS or Android device as well. Shows up in Google Now too.
On Chrome for Desktop the Birthday Doodle shows up on the New Tab page as well, since restoring the "New Tab" to the old look trick does not work anymore and I assume you did not set up a custom Chrome New Tab page yet.
Chrome's New Tab page showing my Birthday Doodle
So that's it. Happy Birthday! :)