Tsiyon Answer - There is a fair amount of confusion behind your question. I've often heard it stated that the 1st Century Jews of Messiah's day kept Passover on the evening of the 15th of Abib. This often stated position is clearly not correct. In fact, the 1st Century Jews kept Passover on the evening of the 14th of Abib/Nisan and then kept the feast of unleavened bread over the 7 days following Passover, from the 15th-21st. Passover and the Feast of Unleavened Bread together continued for eight days and were kept in that fashion until the destruction of the Temple in 70 AD. We know this is true from the testimony of the Jewish priest, Josephus. Notice what he said about this:

In the month of Xanthicus, which is by us called Nisan, and is the beginning of our year, on the fourteenth day of the lunar month, when the sun is in Aries, (for in this month it was that we were delivered from bondage under the Egyptians,) the law ordained that we should every year slay that sacrifice which I before told you we slew when we came out of Egypt, and which was called the Passover; and so we do celebrate this passover in companies, leaving nothing of what we sacrifice till the day following. The feast of unleavened bread succeeds that of the passover, and falls on the fifteenth day of the month, and continues seven days. (Antiquities 3.10.5,  Lev 23)

Concluding thoughts from Eliyahu ben David:

In this week's program we are getting into the spirit of the Passover season by considering some interesting Passovers of history. YHWH has done some wonderful things on Passover, not just the first time, or when Messiah came, but many times. He has also saved up some amazing Passovers for the future too - but that's another program. Anyway, I think hearing about interesting Passovers of history will help to get you into a Passover frame of mind. Also, partners, don't miss the supplement. It has some Josephus quotes in it which you will find to be very interesting reading.

I just love Passover! Each new Passover season is a reminder that we are another year closer to our deliverance from the Gentile nations, when all of the remnant will return home to Israel together in the Second Exodus. This makes Passover as much a rehearsal for the future as it is a memorial of the past. Are you ready to leave Egypt?

This is how you shall eat it: with your waist girded, your shoes on your feet, and your staff in your hand; and you shall eat it in haste: it is the Passover of YHWH. (Exo 12:11)