(Leviticus 23:5) 5 In the first month, on the fourteenth day of the month, between the two evenings is the passover to Jehovah.
(Numbers 28:16) “‘And in the first month, on the fourteenth day of the month, will be Jehovah’s passover.
(Ezekiel 45:21) 21 “‘In the first [month], on the fourteenth day of the month, there should occur for YOU the passover. . . .
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.)