Google has done this for YEARS -- their web designers change the front page graphic all the time, on Thanksgiving Day, Christmas, 4th of July, Boxing Day, anytime they want to. It's not automatic or based on your computer's date/time, it's just how cool and creative Google's website is.
I love Google and has been using it as my fave search engine for years. I never know whether there is a surprise around the Google with its artistic work to celebrate an event.
It is no wonder they are excellent for coming up with creative ideas for improving the service!