The Samaritan Tradition Sheds Light on the Johannine Passion Chronology

An important place in the liturgy of Passover is occupied by the reading of the biblical passages that deal with the sacrifice. These readings and the prayers are led by the high priest. The act of slaughtering takes place when Exodus 12.6 is read. This occurs normally at sunset of Nisan 14. But should this day be a Sabbath, the fire in the "ovens" as well as that of the altar is started only after sunset, ie immediately before the slaughtering of the animals. This delays of course the fleecing of the sheep for which boiling water is needed. If the 15th of Nisan is a Sabbath, the sacrifice takes place in the afternoon of the 14th. [Reinhard Pummer, The Samaritans p. 21, 22]

This is very significant for understanding the discrepencies between the gospel traditions.  The Marcionite Eucharist could not have been understood to be a 'Paschal feast.'  It was instead a wholly new sacrament developed from the 'type' (typos) of the original slaughter (and undoubtedly the events in the Torah which were understood to have fallen on the same day (the slaughter of Abel, the sacrifice of Isaac, even the burial of Joseph).

The Marcionites could not have had Peter and the rest of the 'bad' disciples partake of the holiness of the original Eucharist and I see no evidence that such a narrative was ever in their gospel.  The sacrament was instead undoubtedly understood to have been established by their apostle in memory of his experience as a witness at the Passion.