11 Nissan or Yud Aleph Nissan (Hebrew: י״א נִיסָן Yūʾd ʾAl ef Nī sān lit. 'the eleventh of Nissan') is a holiday on the Chabad-Lubavitch calendar that marks the birthday of the Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson. Rabbi Schneerson was born on April 5, 1902 corresponding to 11 Nissan 5662.

Leading up to the Lubavitcher Rebbe's birth, Chabad-Lubavitch was under the leadership of the Frierdiker Rebbe, Rabbi Yosef Yitzchak Schneersohn. His mother Rebbetzin Chana Schneersohn had two sons already, Dovber Schneerson and Yisroel Aryeh Leib Schneerson.

The Lubavitcher Rebbe was born on April 18 [O.S. April 5] 1902 (11 Nissan, 5662), in the Black Sea port of Nikolaev in the Russian Empire (now Mykolaiv in Ukraine).

The Lubavitcher Rebbe would observe the Frierdiker Rebbe's birthday, 12 Tammuz, by visiting the Ohel (the gravesite of the Frierdiker Rebbe) every year.

During the Rebbe’s leadership, the Rebbe would observe his birthday, 11 Nissan (as well as 25 Adar, the Rebbetzin’s birthday) as a special day every year by going to the Ohel. In 5712 (the Rebbe’s fiftieth birthday), the Rebbe said a maamar to a group of Chassidim, and in 5722 (the Rebbe’s sixtieth birthday) the Rebbe held a special farbrengen. From 5731 onwards, the Rebbe held a farbrengen or said a sicha each year in connection with Yud-Alef Nissan.

From the beginning of the Rebbe's leadership, it was customary for followers of the Rebbe, mainly students of Tomchei Tmimim to arrange a Yechidut (private audience) with the Lubavitcher Rebbe or write a Pan (personal letter) to him on their Hebrew birthday.

After the Lubavitcher Rebbe passed away on June 12, 1994 (3 Tammuz 5754), his followers intensified their celebrations of his birthday, observing it with even greater enthusiasm than when he was alive.

Followers of the Lubavitcher Rebbe commemorate his birthday every year on 11 Nissan, recognizing it as a time for reflection on the Rebbe's contributions to Jewish life and his leadership.