11 What then is our duty, my brethren, for the sake of these things, but to praise and give thanks to God, the King of all? And let us first exclaim in the words of the Psalms, 'Blessed be the Lord, Who has not given us over as a prey to their teeth. ' Let us keep the feast in that way which He has dedicated for us unto salvation— the holy day Easter— so that we may celebrate the feast which is in heaven with the angels. Thus anciently, the people of the Jews, when they came out of affliction into a state of ease, kept the feast, staging a song of praise for their victory.
So also the people in the time of Esther, because they were delivered from the edict of death, kept a feast to the Lord, reckoning it a feast, returning thanks to the Lord, and praising Him for having changed their condition. Therefore let us, performing our vows to the Lord, and confessing our sins, keep the feast to the Lord, in conversation, moral conduct, and manner of life; praising our Lord, Who has chastened us a little, but has not utterly failed nor forsaken us, nor altogether kept silence from us.
For if, having brought us out of the deceitful and famous Egypt of the opponents of Christ, He has caused us to pass through many trials and afflictions, as it were in the wilderness, to His holy Church, so that from hence, according to custom, we can send to you, as well as receive letters from you; on this account especially I both give thanks to God myself, and exhort you to thank Him with me and on my behalf, this being the Apostolic custom, which these opponents of Christ, and the schismatics, wished to put an end to, and to break off. The Lord did not permit it, but both renewed and preserved that which was ordained by Him through the Apostle, so that we may keep the feast together, and together keep holy-day, according to the tradition and commandment of the fathers.
Source: Letters (New Advent)