April was the coldest month.

Or so says Hedonometer.org, a new website that tracks people’s moods across Twitter. Researchers at the site have created an interactive infographic showing the daily happiness averages of tweets over the past five years. The data is derived from a random sample of about 50 million daily tweets, which are mined for commonly used words that are assigned a happiness score. Words such as “laughter,” “happiness” and “love” rank near the top while words like “suicide, “rape” and “terrorist” rank near the bottom.

One look at the graph reveals that the mood of Twitter users has been in general decline since a peak at the beginning of 2009. The saddest day on record, perhaps not surprisingly, is April 15 of this year, the day of the Boston Marathon bombings. A close second is Dec. 14, 2012, the day of the Sandy Hook school massacre. The data can be deceiving, however. As Stephanie Pappas explained in a Thursday report on LiveScience, the day of the shooting in Newtown, Conn., may have actually been the sadder day, despite the fact that it ranks higher on the chart.  

“That’s because the Newtown shooting happened on a Friday, a generally happy day when people otherwise would be tweeting positive vibes,” Pappas wrote. “The Boston bombings happened on a Monday, when unrelated grouchy tweets about returning to work would have driven the average happiness down.” (And not just any Monday -- April 15 is also the dreaded deadline to file your tax returns with the IRS.)

Meanwhile, some of the results are counterintuitive, like the fact that the happiest day of every year is Christmas. That may seem to fly in the face of anecdotal wisdom that people are more depressed around the holidays, but it probably has less to do with people’s actual moods than it does with increased use of words like “merry” and “happy.” In fact, most of the happiness spikes on the chart coincide with major holidays.

Then there are a few downright anomalous results. For instance, the day Osama bin Laden was killed is ranked as one of the saddest days on record, thanks to an increase in the use of words like “death,” “dead” and “killed.” Perhaps more understandable, but still somewhat surprising, is the day that showed the most significant drop in mood: June 25, 2009, the day Michael Jackson died.  

Hedonometer, according to the website, is based on the research of Peter Dodds and Chris Danforth, two applied mathematicians at the University of Vermont’s Complex Systems Center. The site uses technology created by Brian Tivnan and Matt McMahon of Mitre Corporation.

Of all the mini industries that have sprouted up around Twitter over the past few years, sentiment analytics may be the most perplexing. On the one hand, it’s impossible to resist the notion that tracking language across social media can offer a window into national and global moods. On the other hand, anomalies in such data often reveal just how difficult it is to measure context and syntax across a planet full of irrational human beings.

Either way, the infographic is still a lot of fun. Check it out here.