Yud Gimmel Iyar is the day of passing of R' Yisroel Aryeh Leib, third son of the Rabbi and Mekubal R' Levi Yitzchok and Rebbetzin Chana Schneersohn and brother of our Rebbe.
R' Yisroel Aryeh Leib was known by the Chassidim of his time for his unique genius, especially in explaining deep concepts in Chassidus thought.
We have seen a few times when the deep connection between the Rebbe and R' Yisroel Aryeh Leib came out into the open for all to see. Throughout the years, the Rebbe assisted in initiating and coordinating various projects in his memory. On Yud Gimmel Iyar, 5751 the Rebbe spoke an entire Sicha expounding on the name of the Baal Hayahrtzeit.
The Yagdil Torah organization has compiled a publication with Mishnayos, selected stories, and rare correspondents of R' Yisroel Aryeh Leib in honor of Yud Gimmel Iyar.