We observe the Passover on the 14th of Nisan as the day begins—in the evening. When God first instructed the Israelites in Egypt to keep His Passover, He specifically told them to kill the lamb "at twilight" (Exodus 12:6; Leviticus 23:5) and eat it immediately so that "none of it remain[s] until morning" (Exodus 12:10). Keeping the Passover on any part of the 15th of Nisan, blurs the distinction God makes between the Passover and the Days of Unleavened Bread (see Leviticus 23:5-6; Numbers 28:16-17).
In the New Testament, Jesus instructed His disciples to prepare the Passover for the evening hours of the 14th, just as He had commanded Israel in the Old Testament. Paul shows that the Passover is to be observed "on the same night in which He was betrayed" (I Corinthians 11:23-26). There is no instruction to keep it at any other time.