I've written about Charlton's Horn Fair before but I've recently found out a bit more about it.
No one actually knows when it started, one explanation dates it back to the times of King John. It used to take place for three days from St Luke's day (18th of October), the church of St Luke's in Charlton dates back to 1250 which explains the day of the fair and opens the possibility that it really does go back that far.
I've previously touched upon the connection of horns with cuckoldry, there is though an alternate explanation for the horn connection. Apparently the traditional symbol for St Luke is an ox with horns. Many ancient fairs would apparently display some symbol of their being open, and it's easy to imagine that a fair taking place on a saints day would use the symbols of that saint for that purpose. So perhaps that's how it got it's name.
However the story of King John, in which he was caught in the act with the wife of a Charlton miller by the miller himself and thus got permission to hold the fair as compensation, is far juicer and therefore probably became more popular as the years passed.
The fair was opened by the procession from Cuckold's Point for which people apparently dressed up as the miller, his wife and the King. Cross dressing was also apparently the thing to be done for this procession. William Fuller wrote in 1703:
"I remember being there upon Horn Fair day, I was dressed in my landlady's best gown and other women's attire, and to Horn Fair we went, and as we were coming back by water, all the clothes were spoilt by dirty water etc. that was flung on us in an inundation, for which I was obliged to present her with two guineas to make atonement for the damage sustained."
The Victorian's weren't too keen on London's fairs which had a notoriety for unpleasant and lewd behaviour, Charlton's reputation was such that when an law was passed in 1871 that allowed such fairs to be abolished it was one of the first to go.
This additional information comes from the book London Lore by Steve Roud. It's packed with all sorts of tales about London and it's a really enjoyable read, so if you're into this sort of thing it's well worth a look.