WONDERFUL POST LEOLAIA!
This is all such a mess. However, I would like to include a few misconceptions.
1. The only reason why JWs are a day ahead of the Jews this year is because of a different calculation for the lunar date of the 14th.
That is, Jews believe that the 14th falls on Monday, the 18th. This is when they celebrate the Seder meal after sundown, which is the 14th! They are using the calendar date which begins and ends at midnight. However, just as with every sabbath which is dated on a Saturday, we all know the official sabbath begins on Friday after midnight.
Here is a reference where it is clear that the Seder, though celebrated on Monday, the 18th, is still considered the 14th and not the 15th.
The link above clearly shows this.
Thus one misconception by JWs is that they think the Jews celebrate passover on the wrong day. My impression is someone looked at a regular calendar, saw that Passover was dated on the calendar on the 15th and they thought it should be celebrated on the 14th, so they decided to be a day ahead, thinking they were being more Biblically correct. But just because it says passover is held on the 15th, it is not held that night but the previous night on the 14th.
So this is just a misconception of theirs that the Jews celebrate their Seder on the 15th. Technically, they don't, though that appears that way on the calendar. So this represents a cultural misconception by JWs. How so? On a JW calendar the actual date of the passover that evening is given on the calendar. But on a Jewish calendar, the passover would be eaten the day before. In other words, if Jews looked at a JW calendar and saw the Passover/Memorial was on the 17th, they would presume the meal was eaten on the 16th in the evening.
Now of note, JWs used to be smarter than this. They have understood before, for instance, that the 1st day of the month occurred at least 18 hours after the technical new moon, which would have been the earliest that the new moon crescent might have been observed by the naked eye. However, the Full Moon might have been more significant with respect to when Passover was eaten. Thus it is interesting that the technical Full Moon does not occur until 10:44 p.m. EDT (7:44 p.m.. PDT). So if the Full Moon was a consideration for when Passover was to occur, JWs celebrated Passover too early in some time zones. For instance, after sundown around 7:30 p.m. in New York was still 3 hours shy of the technical Full Moon.
By contrast, the Jewish calendar may have assigned the 14th to the day after the Full Moon, that is, after Sunday night and thus on Monday. Even so, in their minds, the Passover Seder is eaten on the 14th after Sundown until midnight. But on their calendars, just as the sabbath is dated on a Saturday, it doesn't reflect that sabbath day actually begins on Friday, after sundown.
So, again, JWs are mistaken that the Jews eat passover on the 15th, that is, the evening of the 15th, which they don't. They eat passover on the evening of the 14th.
2. What else is wrong with this picture is another blind spot for JWs. That is, when the Israelites first left Egypt, they followed the Egyptian custom of changing the calendar date, which is the solar date, at MIDNIGHT. The Seder meal ended at midnight, however, and thus it has always been spoken of as being celebrated on the 14th. The festival of the first day of unfermented cakes, which was a solemn assembly at the temple, was conducted on the 15th.
So this addresses another technicality misunderstood by the witnesses, which is the passover being eaten at the end of the 14th or at the very beginning. Clearly, since Luke 22:7 says the 2 disciples were sent out to prepare for the meal the very day the lambs were killed, and thus Nisan 14th, with the 14th not ending officially until midnight, the meal was eaten after sundown on the 14th at the end of the day. There is no way around this. Thus there is no contradiction with the historical reference by Josephus that the lambs at the temple specifically were killed from 3 p.m. It is also clear that one of the disciples had to take the lamb to the temple to be slaughtered. JWs want us to believe, therefore, that at sundown when they think "between the two evenings" was between sundown and nightfall, that one of those disciples were at the temple getting the lamb killed. No way. Besides that contradicts Luke 22:7. No matter when you decide the 14th actually began since the day they were sent to prepare was before the sunset of the day the meal was eaten, they were sent out on a different day than the meal was eaten. So even if the 14th began at sunset and the 2 disciples were sent out a minue after sunset, rushed to the temple to have the lambs killed before nightfall, it still wouldn't be until after sunset that following day that Jesus and the others arrived. So their claim he sent the disciples out on the 13th is just a blatant lie and contradiction of scripture.
Perhaps, then the key text that we should consider is precisely when the 1st day of unfermented cakes began. As I noted, if the actual date in reference changed at midnight, then the sabbath day of the 1st day of UFC would have begun on the "fourteenth in the evening." Well, that is precisely when the 1st day of UFC is said to begin:
Exodus 12: 18 " In the first month, on the fourteenth day of the month, in the evening YOU are to eat unfermented cakes down till the twenty-first day of the month in the evening."
Of course, things get confused if you change the date at sundown instead of at midnight.
It seems clear Jehovah must have blinded them to all of this, which is why they have become the "man of lawlessness" fulfilling 2 Thess 2:4.
Of course, since it is quite obvious the Israelites left Egypt on the 15th but on the same night that they ate passover on the 14th, that the passover was eaten on the first day of unfermented cakes, which began on the 14th after sundown. But that also meant that passover was eaten on a sabbath day. So Nisan 14th up until sundown is the day of preparation, but after sundown, Nisan 14th until nightfall Nisan 15th is the sabbath day of the first day of UFC. That means, no matter what day of the week Nisan 14th falls on, passover is always eaten on a sabbath day, the first day of UFC, the very same day they leave Egypt.
Thus if you insist upon changing the date at sunset, if the Bible tells you that the Israelites left on the 15th, then the 15th began at sunset which is when the passover was eaten that same night. Thus passover would be eaten on the 15th if you change the date at sunset since the Israelites after midnight were said to have left that same night but it was the 15th.
But that also means, as I've pointed out before, that Jesus could not have died the same day he ate passover because passover always occurs on a sabbath day and impalements were not carried out on a sabbath day. Further, the Bible is explicit that Jesus died on a day of perparation; it was already the 15th when Jesus was arrested after midnight. So this is on top of another lie and false teaching, that Jesus died on Nisan 14th, the same day he ate passover. IMPOSSIBLE.
SUMMARY: The only reason why JWs conduct passover on the 17th instead of the 18th, is not because the Jews celebrate passover on the 15th, but because of the difference between the dating of the lunar calendar. Jews date the 14th on Monday, JWs date the 14th on Sunday. But of note, in favor of the Jews, the technical Full Moon does not begin until 10:44 p.m. EDT. So if that was the significant factor for celebrating passover, that is, DURING the day the Full Moon, then JWs celebrating passover at 7:30 p.m. would have celebrated passover on the wrong day, that is, at a time before the Full Moon actually occurred. Apparently JWs are going by the 1st day of the month occurring at least 18 hours after the technical New Moon which would date the 1st of the lunar month on the 4th, which in turn dates the 14th of Nisan on Sunday the 17th. The Jews must calculate from the 5th, a day later, but that assures that passover occurs after the Full Moon.
Oh well. Since Jesus allegedly arrived in 1914 according to them, it doesn't matter what day they celebrate passover since technically the Memorial should have ended the passover after Jesus' arrival! Further, due to their misunderstanding of Jewish culture and tradition of passover, they have misinterpreted when the Jews celebrate passover. The Jews celebrate passover on the evening of the 14th as mandated by the Bible. JWs misinterpreted the Jewish calendar date for passover with their own. JWs must have presumed the date of passover on a Jewish calendar must have meant it was celebrated that evening as in their case, but it is not; it is celebrated the evening before.
Quite ironically, even if you date the 14th as beginning at sundown in the evening, if you follow Luke 22:7 it would have been that following afternon when the disciples were sent out to prepare for the meal and that afternoon when the lambs would have been killed, which means when Jesus and the others arrived for the meal, it would have been eaten on the 15th after sundown per JW reckoning. The irony is, that that is precisely when the Jews celebrated it as well, Monday night, rather than Sunday night. So clearly, the JWs celebrated the memorial on the wrong evening this year, no matter how you slice it.