The 5th symphony was first performed in Vienna on 22 December 1808. Beethoven conducted a charity concert of his music lasting 4 hours which also premiered the 6th symphony, the 4th piano concerto (played by Beethoven), 2 movements of the mass in C major and his choral fantasy, which was written for the occasion after he concluded the 5th symphony wasn't strong enough to end the programme!
The orchestra, which would have included a large number of amateur musicians, had not been well rehearsed. Playing from badly lit, hand written scores, there were plenty of mistakes. Sections had to be repeated, to the amusement of the audience and anger of the musicians. The event has been described as not so much a concert as an exercise in sight reading. By the beginning of the second half, the audience and orchestra were almost certainly frozen. December in Vienna in the early 19th century was bitterly cold and the theatre poorly heated. It is not difficult to imagine the tired musicians indulging in a glass or two of Christmas cheer during the break and perhaps being less attentive than normal when Beethoven, by then suffering badly from deafness and tinnitus, lifted his baton to signal the start of the most famous bar of music ever written.
We can only guess what the audience made of it. They would certainly have been confused, as the 5th had been wrongly described in the programme as the 6th symphony; but they must have been impressed by an extraordinary night of music.