In 1968, President Lyndon Johnson signed legislation to establish the first-ever Hispanic Heritage Week to honor the contributions and influence of Hispanic Americans in the United States. It began on Sept. 15, marking important independence day anniversaries for a number of countries, including Mexico, Guatemala, Belize, El Salvador, Chile, Costa Rica, Honduras and Nicaragua. The week eventually grew to a month, signed into law by President Ronald Reagan in 1988, extending through October and encompassing Indigenous People's Day/Dia de la Raza (Oct. 12). Nowadays, Hispanic Heritage Month is celebrated from September 15 to October 15.
Source:
Diaz, Ann-Christine. “Making a Mark for Hispanic Heritage Month: In Honor of the Celebration, a Selection of Standout Creative Ideas That Speak to and Give Voice to the Latinx Community.” Advertising Age, vol. 92, no. 14, Oct. 2021, p. 22.