Saint Matthias replaced Judas as the twelfth Apostle, as decribed in the Acts of the Apostles:
Acts 1:15-26 “During those days Peter stood up in the midst of the brothers (there was a group of about one hundred and twenty persons in the one place).
He said, “My brothers, the Scripture had to be fulfilled which the Holy Spirit spoke beforehand through the mouth of David, concerning Judas, who was the guide for those who arrested Jesus. He was numbered among us and was allotted a share in this ministry. He bought a parcel of land with the wages of his iniquity, and falling headlong, he burst open in the middle, and all his insides spilled out. This became known to everyone who lived in Jerusalem, so that the parcel of land was called in their language ‘Akeldama,’ that is, ‘Field of Blood’. For it is written in the Book of Psalms: ‘Let his encampment become desolate, and may no one dwell in it.’ And: ‘May another take his office.’
Therefore, it is necessary that one of the men who accompanied us the whole time the Lord Jesus came and went among us, beginning from the baptism of John until the day on which he was taken up from us, become with us a witness to his resurrection.”
So they proposed two, Joseph called Barsabbas, who was also known as Justus, and Matthias. Then they prayed, “You, Lord, who know the hearts of all, show which one of these two you have chosen to take the place in this apostolic ministry from which Judas turned away to go to his own place.” Then they gave lots to them, and the lot fell upon Matthias, and he was counted with the eleven apostles.”
Saint Matthias was present with the other Apostles at Pentecost. Aside from this, little is known of the rest of Saint Matthias’ life. According to one tradition, he preached for a time in Jerusalem and eventually he was stoned and beheaded. (He is often portrayed holding an axe, the instument of his death.) Another tradition claims he preached the Gospel to the barbarians and cannibals in the interior of Ethiopia and was crucified.
Saint Matthias is the patron of carpenters, tailors, those with smallpox, and in prayers for perseverence and for hope.
His feast was originally celebrated on February 24 (or February 25 in leap years) but in 1969 was moved to May 14 so as to be celebrated outside of Lent.