30 September - Panama Canal Zone Last Day of Operation 1979

The Panama Canal Zone was an unincorporated territory of the United States from 1903 to 1979, centered on the Panama Canal and surrounded by the Republic of Panama. The zone consisted of the canal and an area generally extending five miles (8.0 km) on each side of the centerline, excluding Panama City and Colón, which otherwise would have been partly within the limits of the Zone. Its border spanned three of Panama's provinces. 

In 1904, the Isthmian Canal Convention was proclaimed. In it, the Republic of Panama granted to the United States in perpetuity the use, occupation, and control of a zone of land and land under water for the construction, maintenance, operation, sanitation, and protection of the canal. From 1903 to 1979, the territory was controlled by the United States, which had purchased the land from the private and public owners, built the canal and financed its construction. The Canal Zone was abolished in 1979 on 30 September, as a term of the Torrijos–Carter Treaties two years earlier; the canal itself was later under joint U.S.–Panamanian control until it was fully turned over to Panama in 1999

On September 30, 1979, the Canal Zone ceased to exist, and its stamps consigned to the status of 'dead country'. This has not lessened the attraction its stamps and covers hold for collectors. Happy to let you all know i have a cover of last date of operation .Enjoy the beauty of the cover which is displayed above. 

The cancellation of the cover is of "BALBOA" its a multimodal (ship-to-train) terminal, called the Pacific Terminal, connected to Colón by the Panama Canal Railway.

30 September - Hoover Dam (Boulder Dam) in 1935

Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the U.S. states of Nevada and Arizona. It was constructed between 1931 and 1936 during the Great Depression and was dedicated on September 30, 1935, by President Franklin D. Roosevelt. Its construction was the result of a massive effort involving thousands of workers, and cost over one hundred lives. Originally known as Boulder Dam from 1933, it was officially renamed Hoover Dam for President Herbert Hoover by a joint resolution of Congress in 1947.

Since about 1900, the Black Canyon and nearby Boulder Canyon had been investigated for their potential to support a dam that would control floods, provide irrigation water and produce hydroelectric power. In 1928, Congress authorized the project. The winning bid to build the dam was submitted by a consortium called Six Companies, Inc., which began construction of the dam in early 1931. Such a large concrete structure had never been built before, and some of the techniques were unproven. The torrid summer weather and lack of facilities near the site also presented difficulties. Nevertheless, Six Companies turned the dam over to the federal government on  01 March 1936, more than two years ahead of schedule.

Hoover Dam impounds Lake Mead, the largest reservoir in the United States by volume (when it is full).[6] The dam is located near Boulder City, Nevada, a municipality originally constructed for workers on the construction project, about 30 mi (48 km) southeast of Las Vegas, Nevada. The dam's generators provide power for public and private utilities in Nevada, Arizona, and California. Hoover Dam is a major tourist attraction; nearly a million people tour the dam each year. The heavily traveled U.S. Route 93 (US 93) ran along the dam's crest until October 2010, when the Hoover Dam Bypass opened.