You can observe passover if you are Jewish, but for Christians, the Lord's Passover is in fact what happened on between Holy Thursday and Easter.
Actually the Lord's Passover is what happened on Nissan 14th 30 AD on a Wednesday --- not Thursday or Friday. Otherwise there would not have been three days and three nights between the Crucifixion and the Resurrection.
And Easter Sunday of that week [Nissan 18th] was the Jewish Feast of First Fruits --- coinciding with the Resurrection. So no matter how hard theologians try to avoid it, the Jewish roots are the determining factor of Holy Week and cannot be avoided.