“Between 6 and 7 in the morning,” says Silver:
Flights scheduled to depart in that window arrived just 8.6 minutes late on average. Flights leaving before 6, or between 7 and 8, are nearly as good. But delay times build from there. Through the rest of the morning and the afternoon, for every hour later you depart you can expect an extra minute of delays. Delay times peak at 20.7 minutes — more than twice as long as for early-morning flights — in the block between 6 and 7 p.m. They remain at 20-plus minutes through the 9 p.m. hour. …
Late-arriving aircraft account for the bulk of the difference in the timing of delays. Ever have one of those days when you’re 10 minutes late to your first appointment and never make up the time? Airplanes are the same way. Early in the morning, almost all of them are in position from the previous evening. But there isn’t much slack. Once they’re late, their schedule may be off for the rest of the day.
The Dish 