It was the custom of the Baal Shem Tov to partake of three meals on Acharon shel Pesach, the Last Day of Pesach. The third meal, which took place late in the afternoon, was known as "the Festive Meal of Mashiach," for on this day the radiance of Mashiach is openly revealed.

From the year 5666 (1906) it became customary in Lubavitch for the students of the Tomchei Temimim Yeshivah to eat their Pesach meals together in the study hall. That year the Rebbe Rashab joined the students for the [third] festive meal of the Last Day of Pesach, and directed that each of them be given four cups of wine.


HaYom Yom, p. 47