The Boeing 757, affectionately nicknamed 'the flying pencil,' flew for the first time nearly 42 years ago. Boeing had already experienced significant success with the 737 and 727 and wanted to build a 727-300 but lacked customer interest. At that point, the American manufacturer decided to design a new aircraft from scratch, and the Boeing 757 was born. Originally named the 7N7, the 757 returned to a twin-engine design that was up to 80% more fuel efficient than the 727s.
Over the years, more than 1,000 757s were manufactured, and according to ch-aviation, 501 remain active for 49 operators. In the United States, three airlines still operate the 757: United Airlines, Delta Air Lines, and New Pacific Airlines (Ravn Alaska).
Between United and Delta, 168 757s are active, 112 for Delta and 56 for United. The oldest 757 in United's fleet is a -200 (N58101), delivered on May 12, 1994, and the youngest is a -300 (N56859), delivered on April 27, 2004. Delta's oldest 757 is also a -200 (N649DL), first delivered on May 25, 1989, and the youngest is also a -200 (N823DX) that first operated for Shanghai Airlines until Delta took it in April 2015.

According to Cirium, more than 14,000 757 flights are scheduled in the US this month, 1,564 of which are international. More than 70% of the flights scheduled are Delta flights, but that should be no surprise as its fleet is still very large. Delta has 9,974 flights, United has 4,144, and New Pacific Airways has 34.

Delta
Delta's busiest 757 route is from its hub at Hartsfield Jackson Atlanta International Airport to Orlando International Airport, with up to 15 daily roundtrips (30 flights), totaling 807 flights this month. The 'flying pencil' is also frequently deployed to Fort Lauderdale Hollywood International Airport on up to 13 daily roundtrips (26 flights), for a total of 692 flights. The only other route with more than 600 flights is also to Florida, specifically to Tampa International Airport, where Delta sends the 757 on up to 12 daily roundtrips, totaling 642 flights.