This is the borg's comment as to how it is calculated . NB The Jewish day started at sundown, so it could this year be held anytime between sundown Thursday and sundown Friday. But it is always on the evening after dark. I do not believe that they would move it for convenience or any other reason . It is the only important date in the year.
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rs p. 269 par. 2 Memorial (Lord?s Evening Meal) ***Jehovah?s Witnesses observe the Memorial after sundown on Nisan 14, according to the reckoning of the Jewish calendar that was common in the first century. The Jewish day begins at sundown and extends until the following sundown. So Jesus died on the same Jewish calendar day that he instituted the Memorial. The beginning of the month of Nisan was the sunset after the new moon nearest the spring equinox became visible in Jerusalem. The Memorial date is 14 days thereafter. (Thus the date for the Memorial may not coincide with that of the Passover kept by modern-day Jews. Why not? The start of their calendar months is set to coincide with the astronomical new moon, not the visible new moon over Jerusalem, which may come 18 to 30 hours later. Also, most Jews today keep the Passover on Nisan 15, not on the 14th as did Jesus in harmony with what was stated in the Mosaic Law.)