It is the anniversary of independence for Latin American countries
The term Hispanic or Latino, refers to Puerto Rican, South or Central American, or other Spanish culture or origin regardless of race. People of Spanish, Hispanic and/or Latino origin could identify themselves as Mexican, Mexican American, Chicano, Puerto Rican, Cuban, or another Hispanic, Latino, or Spanish origin.
The United States celebrate it because people recognize the contributions of Hispanic and Latino Americans to the United States and celebrate the group's heritage and culture.