The following comes from C.J. Wright's 1950 Jesus the Revelation of God. He argues for the accuracy of the Gospel of John, notwithstanding his very liberal approach to its origin, redaction, and even theology (he claims John teaches a Socinian-like model of Christology and does not teach the personal preexistence of Jesus!):
According to St.
Mark, the Last Supper took place on the evening of ‘the first day of unleavened
bread, when they sacrificed the passover’ (Mk. 1214). He therefore
holds that the Last Supper was the Passover meal, or at any rate that it took
place at the time of that meal. This is also the view of Matthew (262, 17,
19, 20) and Luke 221,7,13,14). The Fourth Evangelist, on the
other hand, dates the Last Supper on the day before the Passover Feast, as it
clear from vv. 1 and 29 of this chapter and from v. 28 of the 18th chapter. In
the latter passage we are told that the passover was not eaten till the evening
of the day on which Jesus was crucified.
Which day is correct?
There are good grounds for concluding that on this issue the Fourth Evangelist’s
date should be accepted. The first ground is that of intrinsic probability.
Mark himself tells us (142)—see also Mt. 265—that the
Sanhedrin wish to avoid the arrest and death of Jesus on the day of the Feast.
It is difficult to believe that the Procurator would have permitted such a
trial and death on an important festival. There is a second ground, which may
be regarded as an important piece of external evidence. The Eastern
Church has always used leavened bread for the Eucharist: this implies
the tradition of the Fourth Gospel.
If the Fourth
Evangelist’s be accepted, the Crucifixion took place at the time when the Passover
lambs and kids were being sacrificed. We have seen that such symbolism appealed
greatly to the author; and we can believe that he would not have hesitated—if necessary-to
alter a date to suit his symbolical purpose. The probability, however, in this
case, is that the appropriateness of the symbolism was already to hand in the
day on which the Crucifixion took place. Further, since Jesus treated this Last
Supper as a Paschal meal, it is easy to understand why the Synoptic evangelists
came to read it as such. (C.J. Wright, Jesus the Revelation of God: His Mission
and Message According to St. John [London: Hodder and Stoughton, 1950], 283)