Exod. 12:15 and 18 (V)
I diligently investigated in treatises what the most learnedHieronymus, In Aggaeum 2:11-15 (CCSL 76A: 734)
the first day of unleavened bread is the fifteenth moon, the eighth day (of the solemnity) is the twenty-first moon. Again in the Book of Questions: We may observe the computation of Easter from the fourteenth moon, which is the first for us according to the Law. For God made all things whole, therefore it was granted to us to celebrate the Pasch during these seven days, from the fourteenth moon until the twenty-first, so that of these, a certain seven days are completed from the parasceve to the resurrection, so that neither should the thirteenth moon be on the passion, nor the fourteenth or fifteenth on the resurrection, lest Christ's passion be before the first moon according to us and lest his resurrection not be on the first day, on which the world began.’’Ambrosiaster, Liber Quaestionum, q. 83 (CSEL 50: 144)
These matters are in the 86th question. Also in the 91st: The Apostle, [he says,] cannot be deceived,’’Ambrosiaster, Liber Quaestionum, q. 96 (CSEL 50: 170)
who says: Christ, our paschal lamb, has been sacrificed,’’1 Cor. 5:7
which is, of course, not his own word but the word of the Law, as Moses says: And it shall come to pass when your children say to you 'What do you mean by this service?' and you shall say 'It is the sacrifice' of the Lords Pasch.’’Exod. 12:26-27
What more,’’ he says, is necessary as testimony? The Law speaks, the Apostle proves, this remains: that the contradictor be cast out as a liar. For it is clear that the passing over was after the Pasch. Therefore the blood gave safety, not the passing over, because the blood prevents the passing over from doing harm.’’ I feared this and it disturbed me.