This is taken from the upcoming book by Frances Spilman of CatechismClass.com on the 12 Apostles:
There is not a great deal known about Saint Matthias. We do know that he was with Jesus, the Apostles and disciples, since Jesus’ baptism by St. John the Baptist. The other Apostles must have known Matthias well during the three years of Jesus’ ministry. Matthias does not speak in the Scriptures so we never hear his thoughts or feelings. We do not know if he was joyous or fearful about becoming an Apostle. We can surmise that he was a brave man to stay with the Apostles when there was so much uncertainty about the future of the Catholic Church.
Matthias was with the other Apostles when the Holy Spirit came at Pentecost-
“And when the days of the Pentecost were accomplished, they were all together in one place: And suddenly there came a sound from heaven, as of a mighty wind coming, and it filled the whole house where they were sitting. And there appeared to them parted tongues as it were of fire, and it sat upon every one of them: And they were all filled with the Holy Ghost, and they began to speak with divers tongues, according as the Holy Ghost gave them to speak.” (Acts 2:1-4)Matthias would have received the gifts of the Holy Spirit, strengthening him to go out on his missionary work. The picture we get from the Fathers of the Church is that of a courageous missionary – either in Jerusalem or in Cappadocia and on the coasts of the Caspian Sea - who may have been martyred. The only personal information we glean is that Matthias spoke strongly against licentious pleasures so that the soul might grow by faith and knowledge. In this he was the perfect counterbalance against the man he replaced – Judas. Judas betrayed Jesus because he allowed his desires to overrule the faith growing in his soul.
Saint John writes that Judas was a thief (John 12:6) while Saint Matthew reports that Judas betrayed Jesus for money (Matthew 26:14-15). As Pope Pius XI wrote in Ad Catholici Sacerdotii (On the Catholic Priesthood) on December 20, 1935 -
“Judas, an Apostle of Christ, "one of the twelve," as the Evangelists sadly observe, was led down to the abyss of iniquity precisely through the spirit of greed for earthly things.”Matthias’ steadfast adherence to Jesus, his willingness to be a leader during a dangerous and uncertain time and his missionary work all proclaim him worthy of the title of Apostle. Saint Matthias’ feast day is traditionally observed on February 24th. Saint Matthias is listed among the saints in the Roman Canon-
"To us sinners, also, your servants, who hope in your many mercies, deign to grant some share and fellowship with your holy Apostles and Martyrs: John, Stephen, Matthias, Barnabas, Ignatius, Alexander, Marcellinus, Peter, Felicity, Perpetua, Agatha, Lucy, Agnes, Cecilia, Anastasia, and all your Saints."