Updated October 6, 2014
From the About link from The National Hispanic Heritage Month website
Each year, Americans observe National Hispanic Heritage Month from September 15 to October 15, by celebrating the histories, cultures and contributions of American citizens whose ancestors came from Spain, Mexico, the Caribbean and Central and South America.
The observation started in 1968 as Hispanic Heritage Week under President Lyndon Johnson and was expanded by President Ronald Reagan in 1988 to cover a 30-day period starting on September 15 and ending on October 15. It was enacted into law on August 17, 1988, on the approval of Public Law 100-402.
The day of September 15 is significant because it is the anniversary of independence for Latin American countries Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua. In addition, Mexico and Chile celebrate their independence days on September 16 and September18, respectively. Also, Columbus Day or Día de la Raza, which is October 12, falls within this 30 day period.