This is where things get interesting from the historical-critical perspective. None of the Gospels actually specifies the date. All of them talk about the time frame under discussion in terms of the Passover, and the Synoptics mention Unleavened Bread. However, it is important to remember that the Synoptics never say that Jesus died on Passover. Talk of Passover and Unleavened Bread in the Synoptics is always done with respect to the meal we now call the Last Supper. By the time the events of the arrest, trial, death, and burial are recounted, all mention of these festal days is missing.
Precisely which day was Passover? Ex 14:6 directs that the lamb or goat be kept until the 14th day of Nisan, and then “slaughtered between the two twilights.” Lev 23:5-6 is quite clear that the 14th of Nisan is the Passover, while Unleavened Bread is celebrated on the 15th. Num 28:16-17 reads likewise, the 14th is the Passover, the 15th is the “pilgrimage feast.”
According to the Synoptics, the meal Jesus ate on Thursday evening was the paschal meal. But in the Fourth Gospel (Jn 18:28,) the Jewish leadership wish to avoid entering praetorium during the day Friday because to do would cause ritual defilement, precluding participation in the paschal meal that night. Thus, in the Synoptics Jesus dies in the daytime following the paschal meal, while in John he dies in the daytime preceding this same meal.
This means that, technically speaking, the Fourth Gospel records the death of Jesus on Passover, and the Synoptics on the day after Passover, that is, Unleavened Bread. Puting dates to it, then, according to the Synoptics, Jesus died on 15th of Nisan, while in John he died on the 14th. Naturally, this will affect any calculation of the year in which Jesus died, as well.
We could simply stop here, stating our belief that none of the Gospels provide a usable historical record. We could also stop, satisfied that we have determined the date within the limits imposed by the Gospels themselves – and this is where I personally am, most of the time.
Those who continue, however, often try to weigh degrees of plausibility. Folks who favor the Synoptics normally do so because Mark, as the oldest of the Gospels, is supposed to be the most historically reliable. This, however, is actually something that should be demonstrated in each case, not assumed. Folks who favor John’s chronology point to the fact that it is highly unlikely that ALL the events recorded during the daytime period of Jesus’ death in the Synoptics could have been accomplished on a feast day, that is, on Unleavened Bread.
By far, however, the most interesting work on this matter is done by folks who want to eliminate the discrepancy – apologists and harmonizers. Here are some samples:
(2) The Synoptics are not describing a paschal meal. (Well, except for the fact that when they talk about the meal, they keep saying they’re talking about a paschal meal (Lk 22:15)…) And there is a tradition of a meal eaten by members of a religious brotherhood, called Haburoth, but most of the evidence is rabbinic, and not all that helpful in the 1st century.
(3) The Synoptics and John can be re-arranged so that things work out. This is a very old idea. If John did, in fact, intend that we should intercalate his work with that of the Synoptics, he neglected to spell out how this was to be done. These attempts, therefore, normally tell us more about their authors than most anything else.
(4) Jesus was using the solar calendar employed at Qumran. However, there is no indication that Jesus used any other calendar than that used by most everyone else, and this matter of calendars was no trivial issue. Jesus gets accused of many things, but never of having any special regard for the Essenes, or of any hint of irregularity with respect to the calendar.
The earliest written record of the Last Supper is Paul’s account in 1 Cor 11:23-25, which shows that Paul knew of a final meal with the disciples, called the Lord’s Supper. This account is, in wording, actually closest to Luke’s version. In 1 Cor 5:7, Paul also refers to Christ as a paschal lamb, and in 1 Cor 15:20, Paul has Christ as the firstfruits of the dead, indicating that Paul knew of a connection between Jesus and the twin feasts of Passover and Unleavened Bread.
The association of the death of Jesus with Passover and Unleavened Bread is therefore very, very, old. Mark seems to have taken over a traditional association of Jesus’ final meal with a paschal meal, but without altering the rest of the passion narrative to make it fit, hence the absence of references to the Passover once the final narrative movement toward death commences. The rest of the Synoptics have faithfully followed his lead.
John’s narrative, on the other hand, incorporates far more paschal-lamb imagery than the Synoptics, showing that John is aware of the association – but does so throughout his narrative, beginning as early as John 1:29. Such a smooth, well-plotted, use makes exegetes suspicious. On the other hand, John’s version is without the need to support a whirlwind of activity burying Jesus on Unleavened Bread. This does not make John’s date historical, but perhaps, when all is said and done, simply more plausible.