What's interesting about the logo change is that Google's official birthday is somewhat disputed: some often declare the company was born on September 15th 1997, the day the Google.com domain was registered. Meanwhile, Google filed for incorporation in September 1998: the papers were filed on September 4th, but Google has been known to celebrate on both September 7th and September 27th.
To explain the inconsistency, the company's FAQ once famously read: "Google opened its doors in September 1998. The exact date when we celebrate our birthday has moved around over the years, depending on when people feel like having cake." In recent years, Google has taken to changing its logo on September 27th, meaning this has been taken as the "official" date.
If you'd like to learn more about Google's intriguing history, Google it. ;)