Google has a tradition of changing its logo on certain special days to reflect that day's importance.

It's Google's way of reminding us certain dates that we should never have forgotten and also a way of paying tribute to someone who did something important to mankind.
Like for instance, Google has celebrated the 125th birthday of Walter Gropius, the profound German architect that falls today, by changing the Google logo to reflect a series of buildings.

All these 'Google special logos' are designed by a graphic artist named Dennis Hwang.