Apolytikion in the Third Tone
O Holy Apostle Matthias, intercede with the merciful God that He grant unto our souls forgiveness of offences.
Kontakion in the Fourth Tone
Truly, into all the world thy sound hath gone forth as a brightly-beaming sun; and it enlighteneth by grace the Church of all nations on the earth, O wonderworking Matthias, Apostle of Christ.
Synaxarion:
After Judas by transgression fell from his apostleship (Acts 1: 25), and hanging himself out of despair ended his life with a wretched and shameful death (Matt. 27: 5), then, that the number of the Twelve not be lacking, all the disciples gathered in one place after the Ascension of the Savior (the number of men and women being 120), and they chose two men from among them, Joseph, called Barsabas, who was also surnamed Justus, and Matthias, and they set them in the midst. Then they prayed to God and cast lots, "and the lot fell upon Matthias, and he was numbered with the eleven Apostles" (Acts 1: 15-26). And thus, having taken the place of Judas, Matthias fulfilled the work of apostleship and the prophecy concerning Judas, which the Holy Spirit foretold by the mouth of David: "And his bishopric let another take" (Ps. 108:7). After this, it is said, Matthias preached the Gospel in Ethiopia, and completed his life there in martyrdom.
In Orthodoxy his feast is celebrated on August 19th.