Back in the day, 1 Chronicles and 2 Chronicles were one book and they went under the Hebrew name Divrei Hayamin, which means something like "the events of the days." Later, Greek translators renamed it the Paraleipomena or "the things omitted from earlier historical texts." Finally, in the 3rd century CE, a church father named Jerome of Stridonium nicknamed it "The Chronicle of the Whole of Sacred History." (Source The Oxford Bible Commentary, New York: Oxford University Press, 2001. p. 267) And that's the name that stuck.
Oddly enough, neither of the later names really seemed to fit the actual book. Since Chronicles goes over a whole bunch of stories that are already covered in the Bible, they're not about stuff that's been "omitted." And because they're missing a lot of important episodes, they're not actually recording "the whole" of anything. So we're going to side with the original namers on this one and declare that Chronicles is definitely made up of "events" that happened back in the "days."