From the Treasury of Daily Prayer:
The festival of St. Peter and St. Paul is probably the oldest of the saints’ observances (dating from about the middle of the third century). An early tradition held that these two pillars of the New Testament Church were martyred on the same day in Rome during the persecution under Nero. In addition to this joint commemoration of their deaths, both apostles are commemorated separately: Peter on January 18 for his confession of Jesus as the Christ (Matthew 16:13-16) and Paul on January 25 for his conversion (Acts 9:1-19).
The New Testament tells us much about both apostles. Peter was with Jesus from the beginning of His ministry and served as a leader among the disciples. Despite his steadfast faith, Scripture also records some of his failures, such as his rebuke of Jesus (Matthew 16:21-23) and his threefold denial of his Lord (Matthew 26:69-75). Following Jesus’ ascension, Peter continued as a leader in the Church (Acts 1:15; 2:14; 15:7).
Paul, a devout Jew also known as Saul, entered the scene as a persecutor of the Church. Following his miraculous conversion, in which the risen Christ Himself appeared to him, Paul became a powerful preacher of the grace of God. During his three missionary journeys (Acts 13-14; 16-18; 18-21), Paul traveled throughout modern-day Turkey and Greece. The New Testament account of his life ends with Paul under house arrest in Rome (Acts 28:16), though tradition holds that he went on to Spain before returning to Rome.