NOTE: After searching several sites and floundering in confusion about the description of whether the Passover is on Nisan 14 or Nisan 15, I finally settled on a clearly spoken site at
Aish.com called ASK THE RABBI. You can follow this link to see how they explain it. Some of the evaluation of the day of Jesus' "crucifixion" depends on how this is determined. This site declares that the Passover lamb is killed in the daylight hours of 14 Nisan, so it can be eaten after sundown which is then the beginning hours of Nisan 15, the date of Passover as well as the first day of the Feast of Unleavened Bread.