From Dom Guéranger's The Liturgical Year.
In leap-year, the feast of St Mathias is kept on February 25.
An apostle of Jesus Christ, St. Mathias, is one of the blessed choir which the Church would have us honour during the season of Septuagesima. Mathias was one of the first to follow our Saviour; and he was an eye-witness of all His divine actions up to the very day of the Ascension. He was one of the seventy-two disciples; but our Lord had not conferred upon him the dignity of an apostle. And yet, he was to have this great glory, for it was of him that David spoke, when he prophesied that another should take the bishopric left vacant by the apostasy of Judas the traitor. In the interval between Jesus’ Ascension and the descent of the Holy Ghost, the apostolic college had to complete the mystic number fixed by our Lord Himself, so that there might be the twelve on that solemn day, when the Church, filled with the Holy Ghost, was to manifest herself to the Synagogue. The lot fell on Mathias; he shared with his brother-apostles the persecution in Jerusalem, and, when the time came for the ambassadors of Christ to separate, he set out for the countries allotted to him. Tradition tells us that these were Cappadocia and the provinces bordering on the Caspian Sea.
The virtues, labours, and sufferings of St. Mathias have not been handed down to us: this explains the lack of proper lessons on his life, such as we have for the feasts of the rest of the apostles. Clement of Alexandria records in his writings several sayings of our holy apostle. One of these is so very appropriate to the spirit of the present season, that we consider it a duty to quote it. 'It behoves us to combat the flesh, and make use of it, without pampering it by unlawful gratifications. As to the soul, we must develop her power by faith and knowledge.’ How profound is the teaching contained in these few words! Sin has deranged the order which the Creator had established. It gave the outward man such a tendency to grovel in things which degrade him, that the only means left us for the restoration of the image and likeness of God unto which we were created, is the forcible subjection of the body to the spirit. But the spirit itself, that is, the soul, was also impaired by original sin, and her inclinations were made prone to evil; what is to be her protection? Faith and knowledge. Faith humbles her, and then exalts and rewards her; and the reward is knowledge. Here we have a summary of what the Church teaches us during the two seasons of Septuagesima and Lent. Let us thank the holy apostle, on this his feast, for leaving us such a lesson of spiritual wisdom and fortitude. The same traditions, which give us some Blight information regarding the holy life of St. Mathias, tell us that his apostolic labours were rewarded with the palm of martyrdom. Let us celebrate his triumph by the following stanzas, which are taken from the Menææa of the Greeks.
(Die IX. Augusti)
Mathia beate, Eden spiritualis, fontibus divinis ut fluvius inundans scaturisti, et mysticis terram irrigasti rivulis, et illam fructiferam reddidisti; ideo deprecare Dominum ut animabus nostris pacem concedat et magnani misericordiam.
Mathia apostole, divinum replevisti collegium ex quo Judas ceciderat, et divinis sapientum sermonurn tuorum fulgoribus tenebras fugasti idololatriæ, virtute Spiritus sancti; et nunc deprecare Dominum, ut mentibus nostris concedat pacem et magnam misericordiam.
Ut multifrugiferum palmitem te Vitis vera direxit, colentem uvam quæ salutis vinum profundit; illud bibentesqui detinebanturignorantia, erroris temulentiam rejecerunt.
Erroris axes, iniquitatis currus, verbi Dei ipse currus factus, gloriose, in perpetuum contrivisti; et idololatras, et columnas et tempia radicitus divina virtute destruxisti, Trinitatis vero tempia ædificasti clamantia: Populi, superexaltate Christum in sæcula.
Ut spirituale cœlum apparuisti, enarrans gloriam unigeniti Filii Dei ineffabilem, Mathia venerabilis; fulgur Spiritus sancti, piscator errantium, lumen divinæ claritatis, mysteriorum doctor; ipsum in lætitia unanimi voce celebremus.
Amicum te dixit Salvator, Buia obtemperantem mandatis, beate apostole, et ipsius regni hæredem, et cum ipso sedentem in throno in futura terribili die, sapientissime Mathia, collegii duodenari! apostolorum complementum.
Crucis velamine instructus, vitæ sæviens mare trajecisti, beate, et ad requiei portum pervenisti; et nunc lætus cum apostolorum choro judicum altissimo adstare digneris, Dominum pro nobis exorans misericordem.
Lampas aureo nitore fulgens, Spiritus sancti ellychnio ardens, lingua tua apparuit, extranea comburens dogmata, extraneum extinguens ignem, o sapiens Mathia, lucem fulgurans sedentibus in tenebris ignorantiæ.
O blessed Mathias! spiritual Eden! thou didst flow, like a full river, from the divine fountain; thou didst water the earth with thy mystic rivulets, and make it fruitful. Do thou, therefore, beseech the Lord that he grant peace and much mercy to our souls.
O apostle Mathias I thou didst complete the sacred college, from which Judas had fallen; and by the power of the Holy Ghost, thou didst put to flight the darkness of idolatry by the admirable lightnings of thy wise words. Do thou now beseech the Lord that he grant peace and much mercy to our souls.
He that is the true Vine sent thee, a fruitful branch, bearing the grapes that give out the wine of salvation. When they drank it that before were slaves to ignorance, they turned from the drunkenness of error.
Being made, O glorious Mathias, the chariot of God’s word, thou didst break for ever the wheels of error, and the chariots of iniquity. By the divine power, thou didst defeat the idolaters, and destroy the pillars and the temples; but thou didst build up to the Trinity other temples, which echoed with these words: All ye people, praise Christ above all for ever!
O venerable Mathias! thou, like a spiritual firmament, didst proclaim the glory of the only-begotten Son of God. Let us with one glad voice celebrate the praise of this apostle, who was effulgent with the Holy Ghost; he was the fisher of them that had gone astray, the light that reflected the divine brightness, the teacher of the mysteries.
O blessed apostle! the Saviour called thee his friend, because thou didst keep his commandments. Thou art heir to his kingdom, and thou art to sit with him, on a throne, at the last terrible day, O most wise Mathias, who didst complete the number of the apostolic college.
Guided by the sail of the cross, thou, O blessed one, didst pass over the troubled sea of life, and didst reach the haven of rest. Do thou now vouchsafe to join the glad choir of the apostles, and beseech the infinite Judge, that he would show himself a merciful Lord unto us.
Thy tongue was a bright lamp of glittering gold, burning with the flame of the Holy Ghost. Thou didst consume all strange doctrines, thou didst quench all fire that was profane, and to them that sat in the darkness of ignorance, thou, O wise Mathias, didst show a brilliant light.
 Ps. cviii. 8; Acts i 20.
 Acts i. 26.
 Stromat., lib. iii., cap. iv.