11 Faustus needlessly objects to our observance of the passover, taunting us with differing from the Jewish observance: for in the gospel we have the true Lamb, not in shadow, but in substance; and instead of prefiguring the death, we commemorate it daily, and especially in the yearly festival. Thus also the day of our paschal feast does not correspond with the Jewish observance, for we take in the Lord's day, on which Christ rose. And as to the feast of unleavened bread, all Christians sound in the faith keep it, not in the leaven of the old life, that is, of wickedness, but in the truth and sincerity of the faith; not for seven days, but always, as was typified by the number seven, for days are always counted by sevens. And if this observance is somewhat difficult in this world since the way which leads to life is strait and narrow, the future reward is sure; and this difficulty is typified in the bitter herbs, which are a little distasteful.
Source: Reply to Faustus the Manichaean (New Advent)