The Beis Aharon zt’l (p.93:) writes, “The six days [of Pesach] are a preparation for
the seventh day, for the primary day is the seventh day of Pesach, since that is
perfection, the day they said shirah.” Rebbe Mendel of Riminov zt’l said, “I
heard from a gadol, a student of Rebbe Elimelech of Lizensk zy’a, who repeated
in the name of his Rebbe that the time most mesugal for attaining emunah is the
night of Shevi’i shel Pesach.