And they kept the Feast of Unleavened Bread, etc. The Apostle teaches us how we ought to keep this feast spiritually, saying: "Let us therefore feast not with the old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth" (1 Cor. 5). This is to be observed for seven days; because throughout all this world's time, which runs for as many days, we ought to lead our life in sincerity and truth, indeed in the sacrifices and holocausts of all good works. And because our Lord, at the time of the Passover, conquered death, tasted momentarily with the eternal power of resurrection; the celebration of the Passover may be referred typologically to the time of our resurrection; so that the building of the temple designates the present state of the Holy Church, the dedication signifies the future life, which happens in the exultation of the holy souls, when they leave the body. The sacrifice of the Passover hints at the glory of the resurrection, when all the elect, no longer believing in the sacrament, but seeing in reality and truth, will be refreshed by the flesh of the spotless Lamb, that is, of our God and Lord. Hence, in this Passover, it is stated that all the priests and Levites, the entire assembly of the people, all who had flocked to them from the nations, were pure, as if they had all attended as one; for then the Lamb of God truly takes away the sins of the world. And as the Apostle John says: "The blood of Jesus, the Son of God, cleanses us from all sin" (1 John 1). Then there will be true unity when God will be all in all (1 John 1; 1 Cor. 15). Then the true feast of unleavened bread will be celebrated in joy, when no leaven of malice or wickedness remains in the elect, and all will cling to the divine vision in truth and sincerity of heart. And this not in the seven days of the passing world, but in the one day of everlasting life in the courts of the Lord, which is better than thousands in the light of the Holy Spirit, whose sevenfold grace the prophet commends to us (Ps. 84).
Source: Commentary on Ezra and Nehemiah