I just revisited this problem, and have come to the conclusion that it is possible to read the synoptics as though it was not the actual night of the Passover that Jesus ate the Last Supper, but it is a stretch to do so.
The writer of the Gospel of John is plain though, and his having Jesus eat the last supper on the 14th and dead by 3pm on the 14th of Nisan allows for all the events to take place, trials etc, and Jesus be removed fom the cross before passover started after sundown on the 15th.
I think, if I have this right, that by mistake the JW's are doing it about the right time this year, except they think this night April 5th should be Passover night.
Our default position should always be that JW's have got it wrong, at least in the important aspects. I have found that position always works.