St Matthias, Apostle
It is to St. Matthias that He confided the secrets made known to Him in the bosom of His Father (Gospel). Wherefore this holy apostle shares the glory " of the princes whom God has established over souls" (Offertory) and " He shall judge the twelve tribes of Israel " (Communion). Since their infidelity, indeed, "their house has become deserted " (Epistle) and the kingdom of heaven is transferred to the Gentiles who do penance. His name figures in the Canon of the Mass (second list).
Let us ask "God, who has included blessed Matthias in the college of the apostles, to grant us through his intercession" (Collect) that we may "atone for our sins" (Secret), so as to obtain "peace and pardon" (Postcommunion).
Mihi autem nimis honorati sunt amici tui, Deus: nimis confortatus est principatus eorum. * Domine, probasti me, et cognovisti me: tu cognovisti sessionem meam, et resurrectionem meam.
To me Thy friends, O God, are made exceedingly honourable: their principality is exceedingly strengthened. * Lord, Thou hast proved me, and known me; Thou hast known my sitting down and my rising up.
(Psalm 138:17,1-2 from the Introit of Mass).
Deus, qui beatum Matthiam Apostolorum tuorum collegio sociasti: tribue, quaesumus; ut ejus interventione, tuae circa nos pietatis semper viscera sentiamus.
O God, who didst associate blessed Matthias to the company of Thine apostles, grant, we beseech Thee, that by his intercession, we may ever experience Thy tender mercy towards us.
Lesson from the Acts of the Apostles.
In those days, Peter rising up in the midst of the brethren said (now the number of persons together was about hundred and twenty): Men and 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. 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, show 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 1:15-26)
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/10066a.htm