It was Memorial Day is a United States on the last Monday of May (May 25 in 2009). Memorial day is formerly known as Decoration Day, it commemorates U.S. men and women who died while in the military service. First enacted to honor Union soldiers of the American Civil War (it is celebrated near the day of reunification after the Civil War), it was expanded after World War I to include American casualties of any war or military action.
It seems like Google is also showing respect to people who have given their life for the country by displaying a very special yellow ribbon on their homepage like the screenshot above. In the past, certain groups and people criticized Google for not changing their logo on this day, while others have congratulated Google for ignoring the holiday.
The Google homepage also showed ribbons of other colors before. A red ribbon was put up on World AIDS day, and a black ribbon was put up during bomb attacks in Madrid in 2004, and London in 2005.