There are many issues with the way the JW calculate things.
TL;DR: Jesus died on Nisan 14 (according to Jewish and Biblical sources) or 15 (according to John), the JW claim that was the day the last supper happened.
First, it is not know when the last meal actually took place. The Bible contains 2 separate stories and the entire timeline of Jesus’ last meal, transfiguration, betrayal, trial, execution and death is not exact or chronological between the stories.
There are religious reasons to have Jesus death happen on Passover or Passover-Seder but you can’t have the 2-3 last days of his life which started with the last meal map onto a single day as the JW claim.
If according to JW tradition, Jesus last supper was on Nisan 14 (Shabbat meal, co-inciding with Passover Seder), he could not have subsequently traveled (as Shabbat had started), kept his disciples awake, been betrayed that morning (Shabbat would apply to Judas and the priests), stood trial the requisite 2 days before the Sanhedrin (Shabbat+2), then (Shabbat +3) stood before Pilate, convicted to carry the cross all the way to Golgotha (which happened apparently during the day), been executed and required the legs broken in order to have the execution end before the same Shabbat/Passover we started with although this is required if JWs seem to interpret that the meal preparations were for Passover-Seder.
There is almost an entire week worth of events happening there, they didn’t have cars to drive from Mount of Olives between Herod’s Palace to the Temple twice and then to Golgotha, those are some big hikes to go back and forth.
Most early Christians would have held (as Catholics still do) that the last supper was a Tuesday or Wednesday although the communion happened a lot more frequently, they likely did not attach any significance to the exact time and date of the last supper, the church still holds to date that communion, mass, Eucharist etc should not happen between Good Friday and Holy Sunday (starting Nisan 14/15 after sundown depending on the story if you held to the Jewish calendar). The JWs however hold both “mass” and the sole communion they have in the year on that date.