Sir William Smith, John Mee Fuller: [t]he Mishna forbids that a capital offender should be examined in the night, or on the day, before the Sabbath or a feast-day (Sanhedrim, iv. 1). This law is modified by the glosses of Gemara. But if it had been recognised in its obvious meaning by the Jewish rulers, they would have outraged it in as great a degree on the preceeding day (i.e. the 14th) as on the day of holy convocation before the Sabbath. It was also forbidden to administer justice on a high-feast day, or to carry arms (Yom Tob, v. 20). But these prohibitions are expressely distinguished from unconditional precepts, and our reckoned amongst those which may be set aside by circumstances. The members of the Sanhedrin were forbidden to eat any food on the same day after condemning a criminal. Yet we find them intending to "eat the Passover" (John xviii. 28) after pronouncing the sentence (Matt. xxvi. 65, 66).[A Dictionary of the Bible, Vol 2 (William Clows and Sons, 1893) 722.]
Mt 26:18, Mk 14:14, Lk 22:11
But, Don Carson (C. NT Scholar & Prof.; Ph.d in Phil.): But not only does this theory leave the historical contradiction with the Synoptics unaddressed, it appears flimsy even at the theological level. John does not in these chapters draw attention to the slaughter of the lambs, nor does he here refer to Jesus as the true Lamb of God. [...] One would have thought, however, that if this were John’s intent he would have achieved much more dramatic power by inserting this time notice just after v. 16a. [The Gospel According to John (W.B. Eerdmans, 1991) 456.]
Counter-motive: a. John would have stronger motivation not to lie (e.g. like the other gospel authors who also saw Jesus as the true Passover lamb, but nevertheless place his death on the 15th [Mk 14, Mt 26, Lk 22]) a. John be motived not to have Jesus die on the 14th, insofar as that would preclude Jesus' celebrating the Last Supper as a Paschal meal.
[?] Friday: "Day of preparation" is a technical term for Friday. [?] "Day of preparation" refers to Friday twice in the same chapter (John 19:31, 42) [?] "eve of the Passover" is how Jews refer to the day before Passover (not "day of preparation for", which is what Jn keeps using. v19, v31, v42).
Jn 18:28; "so that they... might eat the Passover" refers to the Passover meal (at the beginning of 15 Nisan). this is relevant because they would remain defiled... Jn 18:28 refers to events that occurred on the day Jesus was crucified.
But, so what? Chagigah: "eat the passover" (Jn 18:28) can include eating the Hagigah (and Jn 13 already narrated the eating of the Paschal meal)
i. Alfred Edersheim (Jewish-Christan scholar; Lecturer at Oxford): The Chagigah, which was strictly a peace-offering, might be twofold. This first Chagigah was offered on the 14th of Nisan, the day of the Paschal sacrifice, and formed afterwards part of the Paschal Supper. The second Chagigah was offered on the 15th of Nisan, or the first day of the feast of unleavened bread. It is this second Chagigah which the Jews were afraid they might be unable to eat, if they contracted defilement in the judgment-hall of Pilate (John 18:28). [The Temple and Its Ministry and Services at the Time of Jesus Christ (London, 1874)] i. David Smith: It was not the Paschal supper that they would have been debarred from eating had they entered Pilate's praetorium, but the Chagigah or thankoffering, which consisted usually of a bullock. And not only was the 15th Nisan the day on which the Chagigah should be offered,(cf. Lightfoot on Jn 18:28) but every worshipped had to present it in the Temple in propria persona.(Id. on Mk 15:25) [The Days of His Flesh 2nd (A.C. Armstrong & Son, 1905), 538.] i. Roger Beckwith (C. NT Prof. at Wycliffe Hall, Oxford): The Pharisees would have been very scrupulous about the hagiga duty, and as it involved a peace offering, which necessarily included a sacred meal, they would certainly have wanted to remain ceremonially clean so as to be able to eat it. Even more would this have concerned the chief priests, since a share of every peace offering might be an ox from the herd, rather than a lamb or goat from the flock. [Calendar and Chronology, Jewish and Christian: Biblical, Intertestamental and Patristic Studies (1996, Brill) 295.] i. Andreas Köstenberger (Director of PhD Studies, C. Prof. of NT): The present reference may not merely be to Passover itself but to the Feast of Unleavened Bread, which lasted seven days (note Luke 22:1: “the Feast of Unleavened Bread, called the Passover”; see further commentary at 19:14, 31), and in particular to the feast-offering (hagigah), which was brought on the morning of the first day of the festival (cf. Num. 28:18–19). “Eat the Passover” probably simply means “celebrate the feast” (cf. 2 Chron. 30:21).[Baker Exegetical Commentary: John (Baker Academic, 2004). 542.] i. Donald Carson (C. NT Scholar & Prof.): It is tempting here to understand to eat the Passover to refer, not to the Passover meal itself, but to the continuing Feast of Unleavened Bread, which continued for seven days. In particular, attention may be focused on the ḥagigah, the feast—offering offered on the morning of the first full paschal day (cf. Nu. 28:18–19).