The J.W org. claims something along the lines of taking in to account when the new Moon is visible in Jerusalem, and working from that to when the Passover would have been celebrated. Glibly ignoring the fact that they cannot predict the Weather, certainly a year or more hence. And glibly ignoring the other signs that were taken in to account to decide on the Date of Passover.
They do not actually use the same method as the Jews of the 1st century C.E used, they just do their own thing, as usual. Ignoring Facts, Evidence, and even Scripture.
Sadly none of this bothers any J.W who is a solid believer, I pointed this Info. out to a devout elderly J.W lady some years ago, she flounced off, throwing over her shoulder " I don't CARE if the Memorial is held on the wrong day !".