Today is the 160th Anniversary of the First World's Fair, which took place in The Crystal Palace in Hyde Park, London, United Kingdom in 1851. The first one was titled "Great Exhibition of the Works of Industry of All Nations".
It was the idea of Prince Albert, Queen Victoria's husband. You can learn more about the World's Fair at Wikipedia.
Here is the Google logo jumbo sized, click on it to get it even larger:
Here is how it looks when you mouse over the logo. The logo let's you look at all the festivities with a magnifying glass, with your mouse pointer.
Here is a video of how it works:
I should note that it is also Mother's Day in Hungary, Lithuania, Mozambique, Portugal and Spain. Google has Mother's Day logos there:
There are complaints in a thread that this logo is not more represented on other Google properties but the World's Fair logo is taking precedence.