This is kind of funny. Publishers that run Google AdSense on their sites can determine if there is some sort of public holiday taking place by watching their earnings and clicks trends for the day. One such publisher posted a thread at WebmasterWorld asking;
Was 15th August 2006 a public holiday? Because, my impressions was down by at least 25%.
It turns out that was a National Holiday in Italy, Spain, Germany, Korea, Poland, Austria, Lithuania, France, Belgium and some other countries.
Forum discussion at WebmasterWorld.