Question: United States Army post, Fort Elliott, was named for a casualty of the Battle of Washita River, which occurred on what day?

Context paragraphs:
Fort Lesley J. McNair: Fort Lesley J. McNair is a United States Army post located on the tip of Greenleaf Point, the peninsula that lies at the confluence of the Potomac River and the Anacostia River in Washington, D.C. To the peninsula's west is the Washington Channel, while the Anacostia River is on its south side.  Originally named Washington Arsenal, the fort has been an army post for more than 200 years, third in length of service, after the United States Military Academy at West Point and the Carlisle Barracks.

Fort Gillem: Fort Gillem was a United States Army Post located in Forest Park, Georgia, on the southeast edge of Atlanta in Clayton County.  Founded in 1941, it was a satellite installation of nearby Fort McPherson.  The base houses different supply and support units, including the U.S. Army Criminal Investigation Laboratory and the 3rd MP Group (CID), both units of the United States Army Criminal Investigation Command.  It employs 456 active duty personnel, 1,663 Army reservist, and 1,667 civilians.  In 1973, its 1465 acre were annexed by Forest Park.  The fort was named in the memory of Lieutenant General Alvan Cullom Gillem, Jr..

Louis McLane Hamilton: Louis McLane Hamilton (1844–1868) was an officer in the United States Army during and after the American Civil War.  He served as a captain under General George Armstrong Custer in the Indian Territory in present-day Oklahoma, where he died at the age of 24 while leading a charge in the Battle of Washita River.  He was a grandson of Alexander Hamilton, one of the Founding Fathers of the United States.

Fort Bliss: Fort Bliss is a United States Army post in the U.S. states of New Mexico and Texas, with its headquarters located in El Paso, Texas.  Named in honor of LTC William Bliss (1815-1853), a mathematical genius who was the son-in-law of President Zachary Taylor, Ft. Bliss has an area of about 1700 sqmi ; it is the largest installation in FORSCOM (United States Army Forces Command) and second-largest in the Army overall (the largest being the adjacent White Sands Missile Range).  The portion of the post located in El Paso County, Texas, is a census-designated place with a population of 8,591 as of the time of the 2010 census.  Fort Bliss provides the largest contiguous tract (1500 sqmi ) of restricted airspace in the Continental United States, used for missile and artillery training and testing, and at 992,000 acres boasts the largest maneuver area (ahead of the National Training Center, which has 642,000 acres).

Fort Elliott: Fort Elliott was a United States Army post in the Texas Panhandle, operational between 1875 and 1890 and named for Major Joel H. Elliott, a casualty of the Battle of Washita River in 1868.

Battle of Fort Buchanan: The Battle of Fort Buchanan was an Apache attack on the United States Army post of Old Fort Buchanan in southern Arizona, which occurred on February 17, 1865.  Though a skirmish, it ended with a significant Apache victory when they forced the small garrison of California Volunteers to retreat to the Santa Rita Mountains.  Fort Buchanan was the only American military post conquered during the war against the Chiricahua.

Battle of Washita River: The Battle of Washita River (also called Battle of the Washita or the Washita Massacre) occurred on November 27, 1868 when Lt. Col. George Armstrong Custer’s 7th U.S. Cavalry attacked Black Kettle’s Southern Cheyenne camp on the Washita River (near present-day Cheyenne, Oklahoma), part of a major winter encampment of numerous Native American tribal bands.

Fort Mason: Fort Mason, once known as San Francisco Port of Embarkation, US Army, in San Francisco, California, is a former United States Army post located in the northern Marina District, alongside San Francisco Bay.  Fort Mason served as an Army post for more than 100 years, initially as a coastal defense site and subsequently as a military port facility.  During World War II, it was the principal port for the Pacific campaign.

Fort Ord: Fort Ord is a former United States Army post on Monterey Bay of the Pacific Ocean coast in California, which closed in 1994.  Most of the fort's land now makes up the Fort Ord National Monument, managed by the U.S. Bureau of Land Management as part of the National Landscape Conservation System.  Before construction and official designation as a fort in 1940, the land was used as a maneuver area and field-artillery target range during 1917.  Fort Ord was considered one of the most attractive locations of any U.S. Army post, because of its proximity to the beach and California weather.  The 7th Infantry Division was its main garrison for many years.  When Fort Ord was converted to civilian use, space was set aside for the first nature reserve in the United States created for conservation of an insect, the endangered Smith's blue butterfly.  Additional endangered species are found on Fort Ord including Contra Costa goldfields and the threatened California Tiger Salamander.

Fort Rucker: Fort Rucker is a U.S. Army post located mostly in Dale County, Alabama, United States.  It was named for a Civil War officer, Confederate General Edmund Rucker.  The post is the primary flight training base for Army Aviation and is home to the United States Army Aviation Center of Excellence (USAACE) and the United States Army Aviation Museum.  Small sections of the post also lie in Coffee, Geneva, and Houston counties.  Part of the Dale County section of the base is a census-designated place; its population was 4,636 at the 2010 census.

Supporting paragraphs: Fort Elliott Battle of Washita River
Answer: November 27, 1868