The death of tzadikim is compared to the burning of the Beis HaMikdash. Perhaps, that is why there is such an disproportionate amount of tzadikim that were niftar on this day, Rosh Chodesh of the month when the first and second Beis HaMikdash were destroyed, starting with Ahron HaKohen, the "ohev Shalom v'rodef Shalom", which in turn is related to the churban.
Ahron HaKohen (1272 B.C.E)
Elazar ben Ahron HaKohen
R' Yehoshua Heschel ben R' Elazar (died early 1700's) was the Rov of Dubna after R' Yosef Yoska, the Yesod Yosef
R' Noson Nota ben R' Yosef of Averitch (1785) Talmid of the Baal Shem Tov and father-in-law of the Bas Ayin. He did not sleep on Shabbos because it says "V'shomru Bnei Yisroel es HaShabbos", you should watch the Shabbos.
R' Asher Wallerstein (1837) was the son of R' Aryeh Leib, the Shagas Aryeh,
R' Ahron ben R' Moshe Goldstein (1845) was the Rov of Breslov. He was one of the main students of R' Nachman of Breslov. R' Nachman said that he used his ancestral merits to bring R' Ahron to Breslov and that R' Ahron went to his chuppah with a "clean garment". He and R' Naftali were the two witnesses to R' Nachman's famous promise to pull out of Gehinom anyone who gives tzedaka and says Tikun HaKlali by his kever. His son, R' Tzvi Aryeh succeded him and R' Tzvi's son was R' Nachman of Tcherin.
R' Shmuel ben R' Dovid Tzvi Ehrenfeld (1883), the Chasan Sofer, grandson of the Chasam Sofer
R' Ahron ben R' Chaim Halberstam of Sanz (1903)
R' Menachem Nochum of Kobrin-Bialystok (1918) ben R' Yehudah Leib was a talmid muvhak of the Yesod Avodah of Slonim
R' Shlomo ben R' Ben-Zion Halberstam of Bobov (2000) His father, first wife and several children were murdered by the Nazis y's. He remarried after the war and rebuilt Bobov into one of the largest chassidish communities in the world. His only son to survive the war, R' Naftal Tzvi, sucseeded him and passed away in 2005. There is a book, Nor the Moon by Night, about his experiences during the Holocaust. It was mentioned many times after his passing that it was no coincidence that his yahrzeit is the same as Ahron HaKohen's because he was consumate "ohev Shalom v'rodef Shalom" and was loved by all.