No doubt, if you are thinking through the names of the 12 apostles (Matthew, Mark, Luke, John, James, Peter . . .) you may be thinking, "Where is Matthias?"
The answer to your question is found in the Acts of the Apostles:
In those days Peter stood up among the brethren (the company of persons was in all about a hundred and twenty), and said: "Brethren, the scripture had to be fulfilled, which the Holy Spirit spoke beforehand by the mouth of David, concerning Judas who was guide to those who arrested Jesus. For he was numbered among us, and was allotted his share in this ministry. (Now this man bought a field with the reward of his wickedness; and falling headlong he burst open in the middle and all his bowels gushed out. And it became known to all the inhabitants of Jerusalem, so that the field was called in their language Akeldama, that is, Field of Blood.) For it is written in the Book of Psalms, 'Let his habitation become desolate, and let there be no one to live in it'; and 'His office let another take.'(Acts 1, 15-26 RSV-CE) Saint Matthias was there all along during the whole of Jesus' public ministry. He was not among the first twelve chosen by Christ, but he was nonetheless present from the baptism of John until the Ascension. Praise God for this noble man, chosen by the Lord to replace Judas after his betrayal and death. This singular mention of Mathias, however, is all that we know of him from the New Testament.
So, one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us, beginning from the baptism of John until the day when he was taken up from us--one of these men must become with us a witness to his resurrection." And they put forward two, Joseph called Barasabbas, who was surnamed Justus, and Matthias. And they prayed and said, "Lord, who knowest the hearts of all men, show which one of these two thou hast chosen to take the place in this ministry and apostleship from which Judas turned aside, to go to his own place." And they cast lots for them, and the lot fell on Matthias; and he was enrolled with the eleven apostles.
There are several variable historical accounts of Saint Matthias after his enrollment as one of the twelve. According to Nicephorus, Matthias preached in Judea and then in Aethiopia (made out to be a synonym for the region of Colchis, part of present-day Georgia) and was crucified in Colchis. In fact, a memorial remains in present-day Georgian region of Adjara, in the ruins of the Roman fortress of Gonio, claiming that Saint Matthias is buried there.
Another tradition, from the Synopsis of Dorotheus, claims that Saint Matthias preached among the cannibals in Ethopia (the ancient name for those regions south of Egypt and Libya) where he died. Another tradition tells that Saint Matthias was executed by the Jews in Jerusalem. And, another tradition says that he died of old age in Jerusalem.
Whatever the truth, we know that the eleven trusted in God to choose Matthias to take the place of Judas. Just as each of us today is called to be a witness to Christ in our world, Saint Matthias was called to be a witness too. But, we celebrate Saint Matthias because of the special nature of his calling; he was called not just to be a witness among many, but one of the twelve chosen personally by Christ.
The Feast of Saint Matthias was added the General Roman Calendar in the eleventh century and was usually celebrated on February 24. However, with the change of the General Roman Calendar in 1969, the feast was moved to May 14 to be celebrated more closely in time to the Ascension.
O Glorious Saint Matthias,
in God's design it fell upon you
to take the place of the unfortunate Judas
who betrayed his Master.
You were selected by the twofold sign
of the uprightness of your life and the call of the Holy Spirit.
Obtain for us the grace to practice the same uprightness of life
and to be called by that same Spirit
to wholehearted service of the Church.
Then after a life of zeal and good works
let us be ushered into your company in heaven
to sing forever the praises of Father, Son, and Holy Spirit.