On June 29, 2018, Sweden-based Flight Radar 24 tracked 202,157 flights in a 24-hour period, the most since its launch in 2006. At the peak of the day, there were more than 19,000 aircraft in the air at the same time. Typically, Thursday and Friday are the heaviest travel days, and August is the heaviest month. Christmas Day has the fewest flights of the year.
What I find even more interesting, though, is noting where the airplanes aren’t flying, like over Syria, western Iraq, and eastern Ukraine.
Started by a couple of aviation hobbyists, the site collects data from volunteers who detect Automatic dependent surveillance-broadcast (ADS-B) information that is broadcast in real time by aircraft, including aircraft type, carrier, route, registration, altitude, and speed. Most European carriers broadcast this information, but few American-built airliners do. Flight Radar 24 uses data from the FAA for tracking US flights, but that data is delayed by five minutes. Tracking data can also be blocked by request.