12 Wherefore let us not celebrate the feast after an earthly manner, but as keeping festival in heaven with the angels. Let us glorify the Lord, by chastity, by righteousness, and other virtues. And let us rejoice, not in ourselves, but in the Lord, that we may be inheritors with the saints. Let us keep the feast then, as Moses. Let us watch like David who rose seven times, and in the middle of the night gave thanks for the righteous judgments of God. Let us be early, as he said, 'In the morning I will stand before You, and You will look upon me: in the morning You will hear my voice. ' Let us fast like Daniel; let us pray without ceasing, as Paul commanded; all of us recognising the season of prayer, but especially those who are honourably married; so that having borne witness to these things, and thus having kept the feast, we may be able to enter into the joy of Christ in the kingdom of heaven. But as Israel, when going up to Jerusalem, was first purified in the wilderness, being trained to forget the customs of Egypt, the Word by this typifying to us the holy fast of forty days, let us first be purified and freed from defilement, so that when we depart hence, having been careful of fasting, we may be able to ascend to the upper chamber with the Lord, to sup with Him; and may be partakers of the joy which is in heaven. In no other manner is it possible to go up to Jerusalem, and to eat the Passover, except by observing the fast of forty days.
Source: Letters (New Advent)