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Amid the celebrations of American independence comes sobering news from Northeastern University criminologist James Alan Fox.

July Fourth ranks annually as the most violent day of the year, his research shows.

And the holiday falling on a Friday this year—creating a three-day weekend—is added cause for concern, Fox says.

Since 2006, there have been five mass killings (of four or more victims) by gunfire on July 4, according to the Associated Press/U.S. TODAY/Northeastern University , the most extensive data source on the subject.

The trend is affirmed by the . Since 2013, there have been 66 mass shootings on July 4 resulting in four or more people injured or killed.

Two main causes are driving July Fourth violence, Fox says.

"We know from my research and the research of others that as the temperature rises, violent crime rates go up," says Fox, who presides over the mass killings database. "The reason for that is partially the physiological effects of heat on our emotions and temperament—we have short fuses when it gets hot.

"And when it gets warm, we spend time outdoors interacting with acquaintances, neighbors, strangers—which means there are more opportunities to get into arguments and fights," Fox says. "On July 4, people are not working and kids aren't in school. There is too much free time to kill, maybe literally."

Fox says holiday partying also plays a role.

"At large parties where people gather, they may not know everybody," Fox says. "They get into arguments at parties that are fueled by alcohol, maybe drugs. And if someone has a gun, that argument or fight can turn deadly."

Fox notes that July 5 is the second-most dangerous day, with 61 mass shootings.

"That's because some of the July Fourth parties spill over past midnight into July 5," Fox says.

Of the 10 most violent days of the year, nine are in the heat of summer.

"The one exception is Jan. 1—another day of parties when people are not working," Fox says.

This story is republished courtesy of Northeastern Global News .