This holiday dates back to the very early days of ancient Rome, and was celebrated for almost a thousand years, from the times of Romulus, the founder of the urbs, to the reign of Constantine the Great.
In the Ides of October, October the 15th, at the end of the war season, the people of the city, divided into two groups, the inhabitants of the Subura, and the residents of the Sacra Via, went to the Campus Martius, the place where young people us|d to train to fight. There was a rivalry between the two quarters to see who could breed the best race horse. Each group provided a two horse chariot (biga) for the race in honour of Mars. After the competition the flamen Martialis (priest of Mars) sacrificed the off-horse of the winning team (the one that run more) with a spear. The horse's head was placed in the middle of the two large teams made up of members of both factions.
They fought ferociously over the head. The winner team carried it back, as a bloody trophy, to its part of town, and fixed it to the Regia, the royal palace, in the Forum Romanum, or to the Mamilia tower, where the skull remained till the next year.
Its tail was cut off and dropping blood taken to the Regia. This blood was preserved by the Vestal virgins in the temple for the purpose of being used at the Palilia, the festival of the dies natalitius or birthday of Rome, to be burnt, with the ashes of the calves sacrificed at the festival of Ceres and the shells of beans, to produce a purifying smoke that cleaned the city.
When roman religion ended, this festival finished too and didn't find a new meaning as a Christian festivity, as other pagan festivities did.
-- Rodriguez Morales