Hispanic Heritage Month is the period to recognize the contributions of Hispanic Americans to the United States and to celebrate Hispanic heritage and culture. The observation started in 1968 as Hispanic Heritage Week was approved by President Lyndon Johnson and was expanded by President Ronald Reagan in 1988 to cover a 30-day period starting on September 15th and ending on October 15. It was enacted into law on August 17, 1988 on the approval of Public Law 100-402.

"September 15 was chosen as the starting point for the celebration because it is the anniversary of independence of five Latin American countries: Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua. They all declared independence in 1821. In addition, Mexico, Chile and Belize celebrate their independence days on September 16, September 18 and September 21, respectively.

 

http://en.wikipedia.org/wiki/Hispanic_Heritage_Month