Since I'm listening to it right now on CD, I might as well write about it - Yay !!!
It was 1741 in the summer that Handel (at the peak of his musical prowess but depressed and badly in debt) began setting a Biblical libretto by Charles Jennens to music. He did it at his usual breakneck speed. In just 24 days the first draft of Messiah was complete. Like many of Handel's compositions it borrows liberally from earlier works (both his own and those of others). Tradition has it that Handel wrote the piece while staying as a guest at Jennens' country house in England, although no evidence exists to confirm this. It is thought that the work was completed inside a garden temple, the ruins of which have been preserved and can be visited by fans today.
It was premiered during the following season, in the spring of 1742 as part of a series of charity concerts. Right up to the day of the premiere Messiah was troubled by production difficulties and last-minute rearrangements of the score. The Dean of St Patrick's, Jonathan Swift then had it cancelled entirely for a period. He demanded that it be retitled A Sacred Oratorio and that revenue from the concert be promised to local hospitals for the mentally ill. The premiere finally happened on April 13 at the Music Hall in Dublin. Handel led the choir from the harpsichord. Matthew Dubourq (an old old friend) conducted the orchestra.
Handel conducted Messiah many times and, as was his custom, altered the music to suit the needs of the singers and orchestra he had available to him for each performance. So as a consequence, no single version can be regarded as the "authentic" one. Many more variations and rearrangements were added in subsequent centuries by others. A very notable arrangement was one done by Mozart.