From the Roman Missal
The Feast of Saints Soter and Caius.
Both were Popes a century apart, Soter having succeeded Anicetus in A.D. 161.
They were both martyred on this day during the persecution of the Catholic Church by the Roman Empire during the first 300 years following Christ's ascension. The first 30 successors of Saint Peter (whom we eventually started calling Popes in the early third century, which is just a word meaning "Papa") were all Martyrs.