I recently came across an interesting blog post by the "Anthem Philatelist" describing the history of El Salvador's national anthem. Here's an excerpt:
One of the longest national anthems in existence, the Salvadoran anthem is a typical example of a "Latin American epic anthem". It was adopted as the National Song on September 15, 1879 but did not have official recognition until December 11, 1953.
The Official National Anthem was composed by an Italian composer Juan Aberle who arrived in the country at the end of the 18th Century as the Director of an Opera Company. The lyrics were written by General Juan José Cañas, an inspired poet and distinguished military that took arms with the National Army against filibusters, around the year 1856. It was composed by recommendation of President Dr. Rafael Zaldívar and was sung for the first time on September 15, 1879 at the prior National Palace by children and young students from government and private schools of the capital city, and the anthem was very well recieved after that initial performance.