In those days Peter rising up in the midst of the brethren, said: (now the number of persons together was about an hundred and twenty:) Men, brethren, the scripture must needs be fulfilled, which the Holy Ghost spoke before by the mouth of David concerning Judas, who was the leader of them that apprehended Jesus: Who was numbered with us, and had obtained part of this ministry. And he indeed hath possessed a field of the reward of iniquity, and being hanged, burst asunder in the midst: and all his bowels gushed out. And it became known to all the inhabitants of Jerusalem: so that the same field was called in their tongue, Haceldama, that is to say, The field of blood. For it is written in the book of Psalms: Let their habitation become desolate, and let there be none to dwell therein. And his bishopric let another take. [et episcopatum ejus accipiat alius] Wherefore of these men who have companied with us all the time that the Lord Jesus came in and went out among us, beginning from the baptism of John, until the day wherein he was taken up from us, one of these must be made a witness with us of his resurrection. And they appointed two, Joseph, called Barsabas, who was surnamed Justus, and Matthias. And praying, they said: Thou, Lord, who knowest the hearts of all men, shew whether of these two thou hast chosen, to take the place of this ministry and apostleship, from which Judas hath by transgression fallen, that he might go to his own place. And they gave them lots, and the lot fell upon Matthias, and he was numbered with the eleven apostles. — Acts of the Apostles i.15-26.
The first event after Pentecost, that is attested to, in Scripture is: the election of Matthias into the college of apostles. A number of details are told, to us, of the nascent Church and how Catholic the Church was and is. It is clear that after Jesus ascended to Heaven, that, Peter is the prince of the apostles. He declares the acting of the Third Person (of the Trinity) in the Old Testament. Because of the scandal of Judas, there is a vacancy amongst the apostles and an open bishophric, a sedevacantism. A descriptive grossness of the end of the great betrayer (et diffusa sunt omnia viscera ejus) is described, Judas was one of their number, a successor is called for. Well, beyond the twelve apostles, there had been seventy-two disciples from the beginning of Jesus’s ministry. From these, two candidates were chosen; and the lot fell to Matthias, and somehow the Holy Ghost guided this.
Matthias replaced Judas. And not much can be confirmed with certainty, after the centuries, about Matthias other than that. But, place yourself in his position, it has become your lot to replace a treacher and scoundrel. He was a saint and the church continued.
At one time, his day was 24 February, upon the re-ordering of the calendar, his day has been 14 May, and this year, it is three days after Pentecost. We have finished the first chapter of the Acts, and of the history of the Church.