Something else that strikes me as odd about the JW memorial is the actual date of the observance.
In discussing this, there are two terms that are important to differentiate. --"Day" and "Date."
"Day" is the daylight period from sunrise to sunset. --Roughly 12 hours in the spring.
"Date" is the cardinal number on the calendar which represents a 24 hour period including both "Day and "Night."
JW's observe the memorial right as the Date of the 14th begins. Sometimes, in fact just a few minutes after it begins. In other words, on the 13th Day, the JWs know that they will be celebrating the Memorial that evening. Many of them arrange to get off of work early that day. An explanation of this usually figures very prominantly in the sermon. It is always pointed out that the Jewish calendar Date begins in the evening and now that sunset has occured, it's technically the 14th and therefore okay to begin.
This means that the Day after the JW memorial is still the Date of the 14th and the 15th will not begin until the following evening, typically 24 hours after the JW memorial.
Okay so far?
Now the Bible says that the passover sacrifice was chosen on the 10th day of the month.
Tell the whole community of Israel that on the tenth day of this month each man is to take a lamb for his family, one for each household. (Exodus 12:3)
It also says that the animal was kept until the fourteenth day of the month and then slaughtered at twilight:
Take care of them until the fourteenth day of the month, when all the people of the community of Israel must slaughter them at twilight. (Exodus 12:6)
So at the point when twilight of the 14th gives way to darkness and the sun has fully set, the 15th has begun and Passover is celebrated.
In proof of this, note that the Bible is explicit that the next Day after Passover was the 15th:
The Israelites set out from Rameses on the fifteenth day of the first month, the day after the Passover. They marched out boldly in full view of all the Egyptians (Numbers 33:3)
Or as the New World Translation puts it:
And they proceeded to pull away from Ram´e·ses in the first month, on the fifteenth day of the first month. Directly the day after the passover the sons of Israel went out with uplifted hand before the eyes of all the Egyptians.
Taken together, it is crystal clear that Passover technically was observed on the 15th. --After sundown on the 14th Day. Note that this is 24 hours after the JW's observe the Memorial
The next question would be whether Jesus and the apostles followed this timing (i.e. The Law) or not. On this, the synoptics are pretty much in agreement: (The Johannine gospel is nebulous since Caiaphas seems be eating the Passover meal later -cf. John 18:28)
"On the first day of the Feast of Unleavened Bread, the disciples came to Jesus and asked, "Where do you want us to make preparations for you to eat the Passover?" He replied, "Go into the city to a certain man and tell him, 'The Teacher says: My appointed time is near. I am going to celebrate the Passover with my disciples at your house.' " So the disciples did as Jesus had directed them and prepared the Passover. When evening came, Jesus was reclining at the table with the Twelve" (Matthew 26:17-20)
"On the first day of the Feast of Unleavened Bread, when it was customary to sacrifice the Passover lamb, Jesus' disciples asked him, "Where do you want us to go and make preparations for you to eat the Passover?" So he sent two of his disciples, telling them, "Go into the city, and a man carrying a jar of water will meet you. Follow him. Say to the owner of the house he enters, 'The Teacher asks: Where is my guest room, where I may eat the Passover with my disciples?' 15 He will show you a large upper room, furnished and ready. Make preparations for us there." The disciples left, went into the city and found things just as Jesus had told them. So they prepared the Passover. When evening came, Jesus arrived with the Twelve" (Mark 14:12-17)
"Then came the day of Unleavened Bread on which the Passover lamb had to be sacrificed. Jesus sent Peter and John, saying, "Go and make preparations for us to eat the Passover." "Where do you want us to prepare for it?" they asked. He replied, "As you enter the city, a man carrying a jar of water will meet you. Follow him to the house that he enters, and say to the owner of the house, 'The Teacher asks: Where is the guest room, where I may eat the Passover with my disciples?' He will show you a large upper room, all furnished. Make preparations there." They left and found things just as Jesus had told them. So they prepared the Passover. When the hour came, Jesus and his apostles reclined at the table." (Luke 22:7-14)
The first Day of the Feast of Unleavened Bread is of course, the 14th since this was the day the lamb was sacrificed and the OT is explicit on that point. It also is crystal clear from Jesus statements quoted above that he was speaking prior to sundown on that Day of the 14th. Therefore it is also apparent that Jesus celebrated Passover with the apostles afterh sundown when the Day and Date of the 14th had ended and the Night and Date of the 15th had begun. This is precisely when modern Jews celebrate Passover.
Is this important? I suppose it depends on your perspective. It's not important to me, but you would suppose that to JW's, it would be very important. They spend a sizeable amount of time each year explaining why they (correctly) celebrate the Memorial when they do.