Portal:United States/Anniversaries/September/September 9
- 1776 – The Continental Congress officially names its union of states the United States.
- 1791 – Washington, D.C., the capital of the United States, is named after President George Washington.
- 1850 – California is admitted as the thirty-first U.S. state. (state flag pictured)
- 1850 – The Compromise of 1850 transfers a third of Texas's claimed territory (now parts of Colorado, Kansas, New Mexico, Oklahoma, and Wyoming) to federal control in return for the U.S. federal government assuming $10 million of Texas's pre-annexation debt.
- 1956 – Elvis Presley appears on The Ed Sullivan Show for the first time.
- 1972 – In Kentucky's Mammoth Cave National Park, a Cave Research Foundation exploration and mapping team discovers a link between the Mammoth and Flint Ridge cave systems, making it the longest known cave passageway in the world.