Fr. Mohr asked Franz Gruber to compose the melody with a guitar arrangement. It was several years later that Franz Gruber wrote an arrangement for the organ. Historians who have conducted research in recent years believe that Fr. Mohr wanted a new carol that he could play on his guitar.
There is a legend associated with the carol that says, Fr. Mohr wanted the carol to be sung by the children of the village at the midnight Christmas Eve service, as a surprise for their parents. But in the middle of practicing, the organ broke and not a note would come from it! So the children had to learn the carol only accompanied by a guitar. They learned the carol so well that they could sing it on its own without accompaniment.
However, there are no records to indicate that a children's choir was involved or that the organ was broken!
At Midnight Mass in 1818, Fr. Mohr and Franz Gruber sang each of the six verses with the church choir repeating the last two lines of each verse. Mohr set down the guitar arrangement on paper around 1820 and that is the earliest manuscript that still exists. It is displayed in the Carolino Augusteum Museum in Salzburg. There are a number of manuscripts of various 'Stille Nacht' arrangement that were written by Franz Gruber in later years.
The original words of the song were in German (and it was called 'Stille Nacht! Heilige Nacht') and translated in to English went:
Silent night, holy night,
Bethlehem sleeps, yet what light,
Floats around the heavenly pair;
Songs of angels fills the air.
Strains of heavenly peace.
It's thought that the song might have traveled around the area with an organ repairman, Karl Mauracher, who could have taken an early arrangement with him in about 1820. Then two singing families (like the 'Von Trappes' in the Sound of Music) seem to have discovered the song and performed it as part of their concerts. In December 1832 the Strasser family performed it at a concert in Leipzig and it was first performed in the USA in 1839 by the Rainer family who sang 'Stille Nacht' at the Alexander Hamilton Monument outside Trinity Church in New York City. During this time the tune changed to the one we know and sing today!
It was translated into English in 1863 by John Freeman Young. The carol was sung during the Christmas Truce in the First World War in December 1914 as it was a song that soldiers on both sides knew!
By the time that the carol was famous, Fr Mohr had died. Franz Gruber wrote to music authorities in Berlin saying that he had composed the tune, but no one believed him and it was thought that Haydn, Mozart of Beethoven had written it! But then the 1820 manuscript was found and in the top right corner Fr Mohr had written: 'Melodie von Fr. Xav. Gruber.'.
It's now one of the most, if not the most, recorded songs in the world!
*as found on The History of Christmas Carols