In the Martyrologium Romanum on this day we find that this is the commemoration of St. Onesimus:
1. Commemoratio beati Onesimi, quem sanctus Paulus Apostolus servum fugitivium excepit et in vinculis utpote Christi in fide filium genuit, sicut ipse domino eius Philemoni scripsit.
St. Onesimus (+ 68 AD – means "useful") was a slave. At Colosse he stole from his master Philemon and fled to Rome. In Rome he was converted by St. Paul, who sent him back to his master Onesimus with the letter now in the canon of Scripture. In that letter, the Apostle of the Gentiles asks Philemon to accept Onesimus back and offers even to pay back anything Onesimus stole from him. Onesimus travelled back Philemon in the company of Tychicus, who carried Paul’s Letter to the Colossians.