But the majority of those who are accounted believers are not of this advanced class; but from being either unable or unwilling to keep every day in this manner, they require some sensible memorials to prevent spiritual things from passing altogether away from their minds. It is to this practice of setting apart some days distinct from others, that Paul seems to me to refer in the expression, “part of the feast;” and by these words he indicates that a life in accordance with the divine word consists not “in a part of the feast,” but in one entire and never ceasing festival. Again, compare the festivals, observed among us as these have been described above, with the public feasts of Celsus and the heathen, and say if the former are not much more sacred observances than those feasts in which the lust of the flesh runs riot, and leads to drunkenness and debauchery.
It would be too long for us at present to show why we are required by the law of God to keep its festivals by eating “the bread of affliction,” or “unleavened with bitter herbs,” or why it says, “Humble your souls,” and such like. For it is impossible for man, who is a compound being, in which “the flesh lusts against the Spirit, and the Spirit against the flesh,” to keep the feast with his whole nature; for either he keeps the feast with his spirit and afflicts the body, which through the lust of the flesh is unfit to keep it along with the spirit, or else he keeps it with the body, and the spirit is unable to share in it. But we have for the present said enough on the subject of feasts.
Source: Against Celsus (New Advent)