Born in Africa, St. Monica married a pagan whom she converted by her virtues. Having become a widow, she devoted herself to her son, Augustine, whose mind was corrupted by the errors of Manicheism and his soul by the dissolute manners of Rome. "Night and day she prayed" (Epistle) and "she shed for him more tears than other mothers shed over a coffin."
For above all else she was penetrated by the fear of God (Introit), and she knew that in order to obtain the resurrection of the soul of her son, she had to sacrifice herself (Gospel). She atoned for the sins of Augustine.
"Could you, O Lord so writes Augustine himself, "despise the contrite and humble heart of a chaste and mortified widow? Could you reject the tears of one who asked not for money, no for any temporal thing, but only the salvation of her son's soul?"
"God, in His mercy, accepted the tears of charity of blessed Monica" ((Collect) and these two souls now share in the joy (Communion) of Jesus risen again.
St. Monica died at Ostia, in 387.
Cognovi, Domine, quia aequitas judicia tua, et in veritate tua humiliasti me: confige timore tuo carnes meas, a mandatis tuis timui. * Beati immaculati in via, qui ambulant in lege Domini.
I know, O Lord, that Thy judgements are equity, and in Thy truth Thou hast humbled me: pierce Thou my flesh with Thy fear, I am afraid of Thy judgements. * Blessed are the undefiled in the way, who walk in the law of the Lord.
(Psalm 118:75 and 120:1 from the Introit of Mass).
Deus, maerentium consolator et in te sperantium salus, qui beatae Monicae pias lacrimas in conversione filii sui Augustini misericorditer suscepisti: da nobis utriusque interventu, peccata nostra deplorare, et gratiae tuae indulgentiam invenire.
O God, the comforter of the sorrowful and the salvation of them that put their trust in Thee, who had merciful regard to the loving tears of blessed Monica in bringing about the conversion of her son Augustine: grant that by their united intercession, we may grieve over our sins and attain grace and pardon from Thee.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/10482a.htm