Why are Sts. Cornelius and Cyprian honored on the same day?

The two saints weren't related and died on different days, but lived during the same time period and shared in martyrdom.

Most saints receive their own feast day, but on rare occasions multiple saints are remembered in a joint celebration.

Such is the case for Sts. Cornelius and Cyprian, two saints who are mentioned during the Eucharistic Prayer at Mass.

Martyrs of the 3rd century

St. Cornelius was a pope and guided the Church during the reign of Emperor Decius and Gallus. He was forced into exile soon after election and ruled the Catholic Church from 251-253. He was martyred during June of 253.

St. Cyprian was the bishop of Carthage and died a martyr’s death in 258.

According to the Roman Martyrology, September 14 was the date of St. Cyprian’s death and the day when St. Cornelius’ relics were transferred:

Memorial of the holy martyrs Cornelius, pope, and Cyprian, bishop, of whom on September 14 the deposition of the first and the passion of the second are remembered.

Eventually September 16 became the day to recall their deaths and heroic example. The Roman Martyrology continues by explaining why these two saints were paired together:

[T]oday the Christian world praises them with one voice as witnesses of love for that truth that knows no yielding, which they professed in times of persecution before the Church of God and the world.

Essentially they were viewed as important martyrs of the 3rd century and because their deaths were relatively close together, they were added on the same day to the liturgical calendar.