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.