Concluding means coming to an end where everything materializes in it’s ultimate fashion so Acheron Shel Pesach concluding the Pesach which represents the Geulas Mitzraim the first redemption with Moshe rabenu will conclude with the ultimate redemption with Moshiach . That is why the Haftorah is all about Moshiach and also the Bal Shem Tov established to make a third meal at the end of Pesach and said that there is a shine of Moshiach on that day. לקו״ש כב