April 15 was the saddest day on Twitter in the last five years

April 15 was a sad day on Twitter. In fact, it was the saddest day in the past five years, according to measurements used by a team of researchers who post their work at Hedonometer.org.

Researchers who run the Hedonometer monitor a sample of the data posted on Twitter each day and have assigned a “happiness score” of one to nine to each of approximately 10,000 unique words. An average of those scores shows the happiness level on Twitter for each day.

April 15 was the day of the Boston Marathon bombings, which killed three people and injured more than 250 more near the finish line of the world-class event. On that day, people were tweeting words such as victim, sad and injured, which would be assigned a low happiness score.

Another sad day on Twitter in the past year was Dec. 14, 2012, the day Adam Lanza, 20, shot and killed 26 children and six staff at Sandy Hook Elementary School in Newtown, Connecticut.

The Hedonometer also shows Twitter happiness peaking each year on Christmas Day.

The system does have a few flaws: it only measures tweets in English and it judges sentiment by measuring only certain words. Also, the measurement tools are a challenge for complex emotions. For example, on the day that Osama Bin Laden was killed, the Hedonometer rated a very low level of happiness. “While we do see positive words such as ‘celebration’ appearing, the overall language of the day on Twitter reflected that a very negatively viewed character met a very negative end,” the site explains in its Frequently Asked Questions section.