I have to change my mind on this one. I was thinking that the remembrance applied to what amounts to the inauguration of the new covenant. However, being awake in the middle of the night last night for a bit, a thought occured to me. Jesus commanded his followers to "keep doing this in remembrance of me." (Luke 22:19) What does that mean?
Paul expounds on this, then reiterates "keep doing this in remembrance of me" at1 Cor 11:24,25. He adds something in verse 26: "For as often as YOU eat this loaf and drink this cup, YOU keep proclaiming the death of the Lord, until he arrives." Ah! This is the purpose of the observance. But what does this mean? Good to read scholars' commentaries.
Barnes' Notes says: "Ye do show the Lord's death - You set forth, or exhibit in an impressive manner, the fact that he was put to death; you exhibit the emblems of his broken body and shed blood, and your belief of the fact that he died. This shows that the ordinance was to be so far public as to be a proper showing forth of their belief in the death of the Saviour. It should be public. It is one mode of professing attachment to the Redeemer; and its public observance often has a most impressive effect on those who witness its observance."
http://bible.cc/1_corinthians/11-26.htm
So, there you go. WTS is not necessarily incorrect in either the statement or the date. The date (Nisan 14) was the date that Paul said in 1 Cor 11:23, "in the night in which he was going to be handed over". Whether his death occurred on Nisan 14 or not is not relevant (except WTS' position that we're observing the anniversary of his death... my position there is that we are to observe the meal Jesus said to keep doing on the anniversary of the night he was handed over.)
Just my two cents.