From the cited article...

... "It's the gathering, the free time, the drinking," said James Alan Fox, a criminologist and professor at Northeastern University, who oversees a mass killings database maintained by The Associated Press and USA Today in partnership with the university. ...

School's out for the summer

Mass killings are far more likely to happen at a home and most often the victims are related to the shooter or are a close acquaintance.

When school is out, families are spending more time together, children are often home all day and there's a greater likelihood of more victims when everyone is under one roof, said Jesenia Pizarro, a criminology professor at Arizona State University.

Teenagers also have more idle time on their hands. "It's like the opportunity shifts in the summer," she said.

After two mass shootings hours apart in Dayton, Ohio, left a total of three people dead and eight injured in late June, police said one of the shootings took place at a vacant house where hundreds of teens and young adults had gathered.

"It could have been a lot worse," said Eric Henderson, the city's assistant chief, who pointed out it was the third big party since mid-June where trouble erupted after young people took over a vacant house.

More social events, more drinking

Family reunions, block parties and festivals in the summertime all bring more people together " and create more opportunities for trouble, more so when there's drinking involved. ...