I always noticed the fun Google designs for (mostly) significant holidays, but sometimes had to click on the “Google” to read about what the design was commemorating. Recently, Google released their complete doodle database to the public. I never knew this existed before, but apparently it was very limited. Now, you can browse all the doodles. When I stumbled upon this website, I learned that Google celebrates worldwide holidays and the designs actually change in each country.
The first Google designs were created in 1998, with a commemoration of Burning Man festival on August 30. I don’t remember Google in 1998, but even with the same color scheme, the logo looks very different.

In 1999, Google celebrated Halloween and Thanksgiving in the U.S. and had a snowman in the first “o” for a worldwide season’s greetings.
2000 marked the first year that Google produced significantly more designs, beginning with “Happy New Year.” Since the creation of Google doodles, they have since spread internationally.
Here are some fun international designs:

And here's a YouTube video montage with some of the cooler ones: