The Luach Kolel Chabad says it's customary to pray mincha before lunch on the eighth day of Pesach. Why?
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The source for this is Hayom Yom, entry for 22 Nissan.
R' Michoel Aharon Seligson, who has written extensively on Hayom Yom, suggests (Kovetz Haoros Ubeurim 936, Acrobat page 216) that this is in order to fit in the two daytime meals needed on this day (the regular Yom Tov meal, and "Moshiach's Seudah"). So the kiddush (with matzah balls or something similar, so that it's קידוש במקום סעודה) serves as the regular Yom Tov daytime meal, then Minchah, followed by the full meal (with לחם משנה) which doubles as Moshiach's Seudah.
That said, the Rebbe zt"l once commented (cited in Otzar Minhagei Chabad Nissan-Sivan, p. 228) that "nowadays, by request of the ezras nashim, this is no longer practiced."