From the Treasury of Daily Prayer:

St. Matthew, also known as Levi, identifies himself as a former tax collector, one who was therefore considered unclean, a public sinner, outcast from the Jews. Yet it was such a one as this whom the Lord Jesus called away from his occupation and wealth to become a disciple (Matthew 9:9-13). Not only did Matthew become a disciple of Jesus, he was also called and sent as one of the Lord’s twelve apostles (Matthew 10:2-4). In time, he became the evangelist whose inspired record of the Gospel was granted first place in the ordering of the New Testament. Among the four Gospels, Matthew’s portrays Christ especially as the new and greater Moses, who graciously fulfills the Law and the Prophets (Matthew 5:17) and establishes a new covenant of salvation in and with His own blood (Matthew 26:27-28). Matthew’s Gospel is also well-known and beloved for its record of the visit of the Magi (Matthew 2:1-12); for the Sermon on the Mount, including the Beatitudes and the Our Father (Matthew 5-7); and for the institution of Holy Baptism and the most explicit revelation of the Holy Trinity (Matthew 28:16-20). Tradition is uncertain whether Matthew died naturally or a martyr’s death. In celebrating this festival, we therefore give thanks to God that He has mightily governed and protected His Holy Church through this man who was called and sent  by Christ to serve the sheep of His pastures with the Holy Gospel.