From the Catholic Culture website:
This memorial is in honor of the nameless followers of Christ brutally killed by the mad Emperor Nero as scapegoats for the fire in Rome. The pagan historian Tacitus and St. Clement of Rome tell of a night of horror (August 15, 64 A.D.) when in the imperial parks Christians were put into animal skins and hunted, were brutally attacked, and were made into living torches to light the road for Nero's chariot. From AD 64 to 314 "Christian" was synonymous with "execution victim."
There were Christians in Rome within a dozen or so years after the death of Jesus, though they were not the converts of the "Apostle of the Gentiles" (see Romans 15:20). Paul had not yet visited them at the time he wrote his great letter in A.D. 57-58.
There was a large Jewish population in Rome. Probably as a result of controversy between Jews and Jewish Christians, the Emperor Claudius expelled all Jews from Rome in A.D. 49-50. Suetonius the historian says that the expulsion was due to disturbances in the city "caused by the certain Chrestus" [Christ]. Perhaps many came back after Claudius' death in A.D. 54. Paul's letter was addressed to a church with members from Jewish and gentile backgrounds.
In July of A.D. 64, more than half of Rome was destroyed by fire. Rumor blamed the tragedy on Nero, who wanted to enlarge his palace. He shifted the blame by accusing the Christians. According to the historian Tacitus, a "great multitude" of Christians were put to death because of their "hatred of the human race." Peter and Paul were probably among the victims.
Threatened by an army revolt and condemned to death by the senate, Nero committed suicide in A.D. 68 at the age of thirty-one.
Wherever the Good News of Jesus was preached, it met the same opposition as Jesus did, and many of those who began to follow him shared his suffering and death. But no human force could stop the power of the Spirit unleashed upon the world. The blood of martyrs has always been, and will always be, the seed of Christians.