Feast of Dioscuri was a festival when the ancient Romans worshipped Castor and Pollux, divine twins (according to one version) of Zeus. The cult of Dioscuri came to ancient Rome through the influence of the Greek colonies in southern Italy and won great support because of their similarity to the native Roman twins, sons of Mars.
They had a sincere brotherly love. After the death of Castor (Pollux, unlike Castor, he was immortal) Zeus transformed both into the constellation of the northern sky so that they would remain together forever.
The day of July 15th was reportedly the day when the Dioscuri descended from the heavens to help the Romans win their enemies and personally brought the news of victory to Rome.