San Jacinto Day is the celebration of the Battle of San Jacinto on April 21, 1836. It was the final battle of the Texas Revolution where Texas won its independence from Mexico.
It is an official holiday in the State of Texas. An annual festival, which includes a reenactment, is held on the site of the battle.
In 1836, Texian troops surprised the Mexican army camped here, in San Jacinto. Shouting “Remember the Alamo” and “Remember Goliad,” the rebels routed the larger force in only 18 minutes.
Source: texas.gov