Slate has a great piece which asks. Here are some excerpts:
Few keep a closer watch on Google than the editors of National Review... When Google ignored Memorial Day in 2006, editor-at-large Jonah Goldberg wrote on NRO's Corner, "It's kind of sad. They change their logo for all sorts of holidays and occasions. Just last week they paid tribute to Arthur Conan Doyle's birthday. But Memorial Day doesn't seem to rate anything at all." Last June 6, Lopez sniffed, "So today is the D-Day anniversary. Today is the day RFK died 40 years ago. So Google is celebrating Diego Velazquez's birthday....."
Perhaps the most extreme condemnations come from the editors of WorldNetDaily.com, who have all but accused Google of advancing the cause of godless communism. "Google consistently ignores patriotic American holidays such as Memorial Day and Veterans Day," WorldNetDaily's editors wrote last October, "but today it acknowledged an accomplishment of the communist Soviet Union, which launched the Sputnik satellite fifty years ago."Here one may find a definitive history of Google Doodles.
"If you're going to choose to commemorate some really quite bizarre occasions, and never, never in their history, never once commemorating Memorial Day, which is a very significant holiday in the United States, I think that says something about who Google is," says WorldNetDaily editor Joseph Farah. "By the way," he adds, "I like Google's product. I wish there were another company out there that didn't make me sick to my stomach."Saturday is Flag Day. Will Google do right by America and stand up for Old Glory? Hwang won't say. "The randomness is very important to us," he says. "Otherwise it wouldn't be any fun." You can be sure the right will be watching.
Here's some of interest:

Persian New Year - March 20, 2008

Sputnik - October 4, 2007

Persian New Year - March 21, 2007

Persian New Year - March 20, 2005

Chinese New Year - February 9, 2005

Persian New Year - March 20, 2004

Chinese New Year - January 22, 2004

Chinese New Year - February 1, 2003

Bastille Day, France - July 14, 2002