God told Moses that the Passover Feast should be a perpetual institution. Catholics believe that by giving us his Body and Blood at his celebration of the Passover, Jesus perpetuated the Passover in our Masses.
Exodus Chapter Twelve's account of that first Passover gave two reasons for its being called a pass-over. One was that the diners should be dressed for the road because they were about to pass over onto the road to the Promised Land. The other reason for its being called a pass-over was that death would pass over, not touching, those marked with the blood of the lamb.
Both meanings for its being called a pass-over apply to the Mass. First, we should be mentally dressed for the road, resolving at each Mass to leave slavery to sin behind, as we strike out for heaven. Secondly, eternal death passes over those of us marked with the blood of the Lamb of God.