When should one eat Matzah on Pesach Sheni?
The main custom is to eat Matza during a meal of the daytime, on the 14th of Iyar. (This year, since it’s Erev Shabbos, one should make sure to begin such meal before the 10th Halachic hour of the day).
In some places, however, the Rebbe cites the Minhag also in regards to the night of the 15th (the time when the Korban was eaten in the times of the Beis Hamikdash).
It’s recorded in some places that the Rebbe ate the Matzah on the day of the 14th or – sometimes and – on the night of the 15th. (There were a few exceptions, such as when Pesach Sheini would be on a Taanis Behab, or when the Rebbe went to the Ohel).
[To note also that it’s recorded that on Pesach Sheini 5712 – a Friday, and the day after the passing of the Rebbe’s brother, R. Yisroel Aryeh Leib – the Rebbe washed for a meal and ate from the Matzah that was baked on Erev Pesach (even though generally the Rebbe would not wash for a meal on Erev Shabbos).]
Additionally, in the Sicha of Shabbos Parahas Shmini 5740 (not Muga), it’s recorded that the Rebbe mentioned the different customs on this (such as eating on the night/day/afternoon of the 14th), and stated that those who wish to be Mehader (scrupulous) eat on all these 3 occasions, as well as on the days before and after Pesach Sheini. The implication however is that this was not mentioned as a practical directive.
אג״ק ח״ב ע׳ שנב. שיחת פסח שני תש״מ ותשמ״ג. ש״פ שמיני תש״מ. וראה הנסמן באוצר מנהגי חב״ד ניסן-סיון ע׳ רסח ואילך. מעשה מלך ע׳ 218.