Answer: Happiness

The tool is called Hedonometer, and by aggregating data gathered from Twitter, researchers have identified a way to quantify happiness, while suggesting why people are (or aren't) happy at a given time.

Looking back into 2012, one of that happiest days was Christmas, while a decidedly not-so-happy day was April 15, the day of the Boston Marathon Bombings (click image below to enlarge):

Images courtesy of Heonometer.org.