The midnight hour
A mildly agitated editor comes to me to ask about the precise use of midnight, and I open to The Associated Press Stylebook, which I cite when it suits me, and point out that midnight belongs to the day that is ending, not the day that is beginning. “I believe that that is not so,” he says. I shrug. AP has spoken.
If I had wanted to pile on, I could have pointed out that The New York Times Manual of Style and Usage says the same thing. Or that even Wikipedia—we can treat the Devil as Scripture for our purposes—says that while midnight registers on digital clocks as “a.m.,” the exact “00:00” moment does not in fact belong to the new day.
When you think about the origin of the word, it makes sense. If midnight is the middle of the night, nightfall came on the old day, not the new one.