He means by the first day of the Passover the fourteenth day of the first month, when they throw aside leaven, and were wont to sacrifice, that is, to kill the lamb at even. The Apostle explaining this says, “Christ our Passover is sacrificed for us.” [1 Cor 5:7].
For although He was crucified on the next day, that is, on the fifteenth moon, yet on the night when the lamb was offered up, He committed to His disciples the Mysteries of His Body and Blood, which they were to celebrate, and was seized upon and bound by the Jews; thus He consecrated the beginning of His Sacrifice, that is, of His Passion.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).