Tom Hoopes tries to:

Maybe Google is gathering a worldwide campus to its side to recommit us to the earth, and memorializing its confreres of scientific progress in a relentless dedication to a godless project that sees its great ritualistic self-expression in dancing naked around a giant bonfire in Nevada.
Or maybe Google is just being cute.
Sometimes it’s hard to tell which.
“People seem to really look forward to going to our homepage and seeing what new doodle is on there,” Google’s [lead logo designer]Lopez wrote to me. “Some users have described how they started their own Google doodle collection, and some simply tell us how the logos bring them smiles through the day.”
Here’s a partial transcript of my interview with Lopez:
Me: “How many complaints do you get regarding the selection of holidays?”
Google: “[A]s you may imagine, it’s difficult for us to choose which events to celebrate on our site. We have a long list of holidays that we’d like to celebrate in the future. We have to balance this rotating calendar with the need to maintain the consistency of the Google homepage. Some holidays that we haven’t celebrated in the past will be rotated into our holiday doodles for future years.”
Me: “Can you give me a run-down of what holidays are coming up that will get logos?”
Google: “Our users enjoy being greeted with a surprise doodle when they come to the Google homepage so I’m unable to share future doodles we have planned.”
Me: “What is your policy regarding religious holidays?”
Google: “See my answer to your 1st question.”
In other words, Google doodles where Google chooses.