What is the X in X-mas?
nickname of Franciscan Mission San Antonio de Valeroin (begun 1718, dissolved 1793) in San Antonio, Texas; American Spanish, literally "poplar" (in New Spain, also "cottonwood"), from alno "the black poplar," from Latin alnus "alder" (cf. alder).
Perhaps so called in reference to trees growing nearby (cf. Alamogordo, New Mexico, literally "big poplar," and Spanish alameda "a public walk with a row of trees on each side"); but the popular name seems to date from the period 1803-13, when the old mission was the base for a Spanish cavalry company from the Mexican town of Alamo de Parras in Nueva Vizcaya.
A fort, once a chapel, in San Antonio, Texas, where a group of Americans made a heroic stand against a much larger Mexican force in 1836, during the war for Texan independence from Mexico. The Mexicans, under General Santa Anna, besieged the Alamo and eventually killed all of the defenders, including Davy Crockett.
Note: Rallying under the cry “Remember the Alamo!”, Texans later forced the Mexicans to recognize the independent republic of Texas.