Each year on 15 July, the feast day of the Dioscuri, the Roman equestrians used to parade through the streets of Rome in an elaborate spectacle, called “Transvectio equitum” (review of the cavalry). The procession began at the temple of Mars, then stopped at the Temple of Castor and Pollux in the Forum Romanum and continued on to the Temple of Iuppiter Optimus Maximus on the Capitoline. The religious rite traced its origins to a battle when the Dioscuri fought on the side of the Romans.
The Dioscuri (Castor and Pollux) were the children of Leda and either Zeus, the king of the gods, or Tyndareus, Leda’s mortal husband and the king of Sparta. Castor was the son of Tyndareus and thus was mortal, while Pollux was the son of Zeus, who had seduced Leda in the guise of a swan. They are sometimes said to have been born from an egg, along with their twin sisters Helen of Troy and Clytemnestra.
Both brothers were fine horsemen, so they were especially attractive to the Roman equites and to the cavalry.
After Castor was killed Zeus gave Pollux the choice between spending all his time on Olympus or giving half his immortality to his mortal brother. Pollux decided to share his immortality so that they could alternate between Olympus and Hades. In his Odyssey Homer explains that the twins interchanged each day, one being alive, the other dead and then vice-versa the next day.
In one version of the story, they became the two brightest stars in the constellation Gemini or The Twins (though several different pairs are associated with the constellation).
Even after the rise of Christianity, the Dioscuri continued to be venerated and in
some instances they appear to have been absorbed into a Christian framework. However, the Church rejected their immortality and replaced them with equivalent Christian pairs. Therefore, Saints Peter and Paul were adopted in their place as patrons of travellers, and Saints Cosmas and Damian took over their function as healers.