From Rabbi Chaim Fogelman as printed in the OK’s Kosher Spirit magazine (spring 2017)
We don�t find any other Yom Tov which has a special name denoting the last day � there�s no Acharon shel Sukkos or Acharon shel Shavuos.
The prophet Yirmiyahu says, �Therefore, behold, days are coming, says the L-rd, when it will no longer be said, As the L-rd lives who brought Israel up from the land of Egypt.� (Yirmiyahu 16:14)
We call the final day of Pesach �acharon� (the last) because we demonstrate our faith that this Pesach will be the last Pesach in exile. When Moshiach comes, we won�t celebrate Pesach as we do now, which is why we refer specifically to the last day as the Final Day of Pesach.