Mass shootings are not a daily event in the US—they're more common than that. The Washington Post reports that according to the crowd-sourced Mass Shooting Tracker, there have been at least 355 mass shootings in the US so far in 2015, which works out to more than one per day. Some days passed without mass shootings, which the tracker defines as shooting incidents in which at least four people are killed or injured, while four days had as many as five mass shootings. The Post notes that the San Bernardino massacre wasn't even the first mass shooting that day: A woman was killed and three men were injured in an early-morning incident in Savannah, Ga., involving two shooters, reports the Atlanta Journal-Constitution.
FiveThirtyEight finds that while the tracker uses a fairly broad definition of mass shootings, evidence from other sources shows that there were "more total mass shooting incidents and deaths in the 11 years starting with 2005 than there were in the previous 23 years combined," and the FBI has also found a rise in what it calls "active shooter" incidents, with shooters "actively engaged in killing or attempting to kill people in a confined and populated area." FiveThirtyEight notes, however, that murders committed with firearms are down from a decade ago, as is the overall murder rate. (A gun sales record was broken on Black Friday this year.)