I think this article will help you understand.
Note: Jack originally published this article in 2003. It remains one of the most popular on this site, and for good reason. It’s a great example of what we love about his studies: taking God at His Word and not relying on human logic or tradition. This study has been an eye opener for so many,
gracethrufaith.com
(I will point out that they (GrThFa) are incorrect when they say that "the next day begins at sundown," as it was pointed out by Dr. McMurtry that the next day starts at darkness, not sundown, but other than that, the article provides a proper timeline.)
In other words, they didn't kill the Passover lamb a day early. Passover itself is a day prior to when you believe it took place. Passover is on 14 Nisan, not 15 Nisan, regardless of which days of the week they happen to be that year.
That year, 15 Nisan fell on Thursday night to Friday sundown. And 14 Nisan was Wednesday night to Thursday sundown (which follows with Genesis 1's "evening and morning were the next day," and with Leviticus 23's details given by God (and Exodus 12)).
Which means 10 Nisan was when the lamb was chosen. (Palm Sunday)
And 11, 12, and 13 Nisan was inspecting the lamb for blemishes. (As the article describes, there was some pretty fierce questioning of Jesus until the end of the 13th (which was Wednesday!) at which point Jesus takes them to the upper room! Where on 14 Nisan (Wednesday night), Jesus eats His final meal with the Disciples, before he is taken to be slaughtered as the Lamb.
It just fits!
Passover is the full day. That means that it begins at darkness on what we consider the previous day, and continues until sundown of the current day. Jesus and His disciples ate the passover meal after darkness had fallen on Wednesday, but that's according to our calendar. According to the Jewish calendar, it was already 14 Nisan, Passover. As pointed out in John 18:28, the Jews who brought Jesus to the Romans had not YET eaten passover, and still wanted to be able to do so, thus did not enter the Governor's headquarters.
Jesus was on the cross, and the Bible tells us that the sabbaths were drawing near, so they needed to get his body down quickly. Not just one, but multiple. The greek word in Matthew 28:1 and Luke 23:54 is "sabbaton," the plural form of sabbath.
Friday that week was a High Sabbath (which are associated with dates on the calendar given to God by the Jews, not with days of the week). He thus could not have been crucified on Friday, 15 Nisan, because that was Unleavened Bread, a high Sabbath, but instead on 14 Nisan, which started Wednesday night and continued into Thursday. Passover is not a sabbath day.
The answer to your "defeater," Idolater, of "why did Jesus keep the Passover early" is, quite simply, He didn't.
He and His disciples ate the Passover meal on the day of Passover, what would have been Wednesday night, because Jesus wanted to have one last meal with them before being crucified in the morning, then dying at 3 pm, and buried a few hours later.
Jesus ate the Passover, was betrayed at Gethsemane, was brought before Pontius Pilate to be questioned, was denied by Peter, was rejected by the Jews in favor of Barabbas, was led to the cross, died on the cross, and was buried in the tomb, all on Passover, 14 Nisan, the start of day one in the tomb. 15 Nisan the start of day two. 16 Nisan the start of day three. 17 Nisan before dawn, Jesus rose ON the third day since his burial.
Edit: Changed days of the week to Jewish calendar days to make it clearer, and added clarification on why they didn't kill the passover a day early.