The last day of Pesach aka Acharon Shel Pesach or Moshiach's festival is celebrated specifically on the last day of Pesach is not merely because Moshiach will redeem us from the last exile. Being last has a significance beyond mere numerical order, for that which is last performs a unique function. When the Jews journeyed in the desert after leaving Egypt, they marched in a specific order, divided into four camps. The last to march was the camp lead by Dan, which included the tribes of Naftali and Asher, described by Torah as "ma'asaf l'chol hamachanos" — "gatherer of all the camps."
B'nei Asher encamped next to Shevet Dan, on the north side of the Mobile Mishkan. Included in their camp was Shevet Naftali. All together, the camp of Dan included the shevatim of Dan, Asher, and Naftali. Together these three shevatim secure the rear of Israel as they march. The rear guard is, perhaps, the most critical position. These three shevatim are responsible for ensuring Shevatim Israel are not surprise attacked from the rear. As they march, Israel could be vulnerable to attack. The fiercest and toughest of warriors, are primarily placed in the rear guard, in order to prevent an Amalekite rear attack. These shevatim, thus, are the last to break camp, and likely the last to pitch camp. They are exposed during the march, and depended upon to prevent any surprise attack.Source:
Rashi explains this as meaning that "The tribe of Dan...would journey last, and whoever would lose anything, it would be restored to him." Just as the tribe of Dan, Naftali, and Asher restored lost articles to their owners, so the last day of Pesach provides a Jew with the opportunity to rectify omissions in the service of Pesach, and thereby regain what is rightfully theirs.
Sichah of the Lubavitcher Rebbe, Acharon Shel Pesach, 5742
