That could be their least busy day, for all we know. Weekends are busier than weekdays for airline travel.

But 3am is when a big chunk of planes are parked at gates awaiting the start of the flight day (yes, there are red-eye flights and international but they are a minority.)

Overnight periods are when the air travel system largely resets itself from each day's cascading delays, mechanical failures and bad weather.

Friday and Sunday are the busiest days. Business and pleasure overlap. Sat is mostly pleasure. Mon-Thu are mostly business.