1 Iyar
Reb Abba Shaul, one of the Talmudic gedolim
R' Yosef Yoska ben R' Yehudah Yudel, Yesod Yosef, rebbe of the Kav Hayosher
R' Tzvi Hirsch ben R' Yaakov Ashkenazi, the Chacham Tzvi (1718)
R' Menachem Mendel of Vitebsk (1788) ben R' Moshe. Together with R' Avrohom Kalisker, they were the first Rebbes to move to Eretz Yisroel. Both of their Torah can be found in Pri HaAretz. For more, see here and here.
R' Shmuel Shmelke Halevi Horowitz of Nikolsburg (1778) ben R' Tzvi Hirsh of Chortkov Known as "the Rebbe Reb Shmelke". See here for more on the Rebbe, Reb Shmelke.
R' Avrohom of Slonim-Baranovich ben R' Shmuel, the Beis Avrohom (1933)
R' Akiva Yosef Schlesinger, author of Lev HaIvri (1922)
R' Moshe Shmuel ben R' Aryeh Shapira, Rosh Yeshiva of Yeshivas Beer Yaakov (2006); descendent of the Netziv
2 Iyar
R' Moshe Zakan Mazuz of Djerba (1915)
R' Avrohom Badush of Mexico, author of Me'oros Avrohom
3 Iyar
Maharal Tzintz of Plotzk, R' Aryeh Leib ben Reb Moshe (1833) - Wrote many
sforim and said he would be an advocate for anyone that helps print his
sforim.
Reb Shayale Kerestirer, Reb Yeshaya ben Reb Moshe Shteiner (1925) Talmid of
the Divrei Chaim of Sanz and the Ach Pri Tevua of Liska. Was known for the
yeshuas he helped bring. His picture is one of the most popular ones around -
it is known as a segula to keep mice away.
4 Iyar
R' Yaakov ben R' Ahron Sasportas, Rov of Amsterdam and antagonist of Shabtai Tzvi (1695)
R' Yosef Dov ben R' Yitzchok Zev Halevi Soloveitchik of Brisk, the Beis Halevi (1892)
R' Shmuel Eliyahu ben R' Shaul Yedidya Elazer Taub, Modzhitzer Rebbe (1984)
5 Iyar
R' Yeshaya Pick, author of Hagaos to Mesores HaShas (1799)
R' Moshe Zorach Eidelitz of Prague, author of Ohr La'yeshorim, Berurei Hamidos and Meleches Machsheves (1780)
R' Meir Auerbach, Imrei Binah, Rov of Yerushalayim (1878)
R' Chaim Eliezer ben R' Boruch Rabinowitz, Yampola Rebbe (1916) He was the first Chassidishe Rebbe in America. Descendent of the Magid of Zlotchov and the Baal Shem Tov. The present Skolya Rebbe, who is his great-grandson, along with a large group of his chassidim and many others go to daven at the kever in Queens, at Mt. Judah Cemetery, on the yahrzeit.
6 Iyar
R' Levi bar Gershom, the Ralbag (1344)
R' Yosef Meir ben R' Shmuel Weiss, First Spinka Rebbe, Imrei Yosef (1909)
Talmid of the Sar Sholom of Belz, Divrei Chaim, Tzemach Tzadik of
Vizhnitz and R' Yitzchok Aizik of Zhidichov
R' Dov Berish Zeitlyn of Vilna (1920)
R' Yaakov Chaim Ben R' Yisroel Perlow, Stoliner Rebbe, was Rebbe in Detroit
(1946)
R' Menachem Mendel ben R' Avrohom Sholom Halberstam, Stropkover Rebbe (1955)
7 Iyar
Yahrzeit of R' Shlomo Ephraim of Lunshitz, ben R' Ahron, Kli Yakar (1619)
(The yahrzeit is recorded in some places as 7 Adar II. According to the bio in one of the newer printings of Kli Yakar, this is a mistake that was made on the new matzeiva and 7 Iyar is correct. I could see how this mistake was made Iyar rubbed out could look like Adar.)
8 Iyar
R' Yerachmiel Rabinowitz of Peshischa (1831) son of the Yid HaKodosh, R' Yaakov Yitzchok
R' Moshe Mordechai Twersky of Lublin-Trisk HY"D(1943) ben R' Yaakov Aryeh Leib of Trisk
R' Rephoel Binyomin Levine, Rosh Yeshiva of Beis Aryeh-Yerushalayim (2002) son of R' Aryeh Leib, the famous "Tzadik of Yerushalaim"
9 Iyar
R' Avigdor ben Yitzchok Kara (1439) Mekubal and Dayan in Prague, survivor of the the pogrom of Easter 1389 that killed 3000 Jews.
R' Moshe of Shatz ben R' Boruch of Vizhnitz (1926)
R' Yissachar Chaim Abulafia (1781)
R' Shalom Mashash, Divrei Shalom
R' Moshe Hager, the Rosh Yeshiva of Seret-Vizhnitz (1999) son of the
Seret-Vizhnitz Rebbe, the Mekor Boruch, R' Boruch
10 Iyar
Eli HaKohen and his sons, Chofni and Pinchas (890 B.C.E.)
R' Yitzchok ben Yaakov HaKohen Alfasi, the Rif, one of the greatest and most well known Rishonim
R' Yitzchak Issac Yehuda Yechiel Safrin of Komarna (1874), author of Heichal Habracha, Shulchan Hatahor, Zohar Chai, Nesiv Mitzvosecha, Notzer Chesed and other seforim. Son of R' Alexander Sender of Komarna
R' Dovid Twersky of Tolna ben R' Mordechai of Chernobyl (1882)
R' Hillel Lichtenstein of Kolomei, Hungary (1891), talmid of the Chasam Sofer, vigorously fought against the slightest deviation from Yiddishkeit
R' Yosef Teumim, the Pri Megadim
R' Meir ben Tzvi Hirsh Margolies, Meir Nesivim, talmid of the Baal Shem Tov
11 Iyar
R' Eliyahu Shema HaLevi (1804) Av Beis Din in Eretz Yisroel, wrote sefer Korbon Isha
R' Naftali Tzvi of Ropshitz ben R' Menachem Mendel (1827), wrote sefer Zera Kodesh, talmid of the Rebbe, Reb Elimelech of Lizhensk, Reb Mendel of Rimanov and the Chozeh of Lublin See Reb Naftali Ropshitzer On Yichus
R' Yitzchak of Radvil (1832), son of R' Yechiel Michel, the Magid of Zlotchov, his teaching are in sefer Ohr Yitzchok
R' Yehudah Tzvi of Stretin (1844) ben R' Shmuel Zanvil. He was a talmid of R' Aron Leib of Premishlan and talmid muvhak of R' Uri "the Saraf" of Strelisk. He wrote Degel Machane Yehudah.
12 Iyar
R' Masoud Abuchatzeira, son of R' Yaakov - the Abir Yaakov, and the father of Baba Sali
14 Iyar
Rebbi Meir Baal Haness, See Pesach Sheni - Yom Hilula Of Rebbi Meir Baal HaNess Segulas & Minhagim
Rebbi Yehudah bar Iloy, Tanna
R' Akiva (ben Yaakov) of Frankfurt. There is also a minhag to give tzedaka for the neshama of Reb Akiva Frankfurter, in order to find lost objects.
R' Yehudah Tzvi (the second) ben Reb Uri of Stretin (1907)
15 Iyar
R' Chaim Meir Yechiel Shapira, "the Saraph" (fiery angel) of Moglenitz (1849) He was a descendent, ben acher ben, from the Megaleh Amukos and a Son-in-law of R' Elazar of Lizensk. His father, R' Avi Ezri Zelig was the son-in-law of the Magid of Koznitz. He was raised and taught by his maternal grandfather, the Koznitzer Maggid. He was a talmid of the rebbes of Lublin, Apta, and Peshischah. His son was R' Elimelech of Grodzensk, the father of the Piazeczna Rebbe HY"D.
16 Iyar
R' Meir ben R' Gedalia, Maharam of Lublin, Rav in Cracow and Lemberg, wrote Meir Einei Chachomim - Maharam on Shas (1616)
R' Yechiel Michel ben R' Avrohom Yitzchok Feinstein of Bnei Brak (2003), son in-law of the Brisker Rav and nephew of R' Moshe Feinstein
17 Iyar
R' Yechezkel ben Yehuda HaLevi Landau of Prague, the Nodah BeYehudah (1793) See: Noda BeYehudah - The Most Important Tzadik In The History Of Chassidus ?
R' Moshe Chaim Ephraim ben R' Yechiel of Sudlikov (1800) See: The Bitul of the Degel Machane Ephraim
R' Mordechai ben Yochonon Twersky of Rachmastrivka (1921) and his son R' Pinchas of Ostila HY"D (exact death date is unknown, sometime in 1943, so it is observed with his fathers)
18 Iyar
Rebbi Shimon bar Yochai, See: Lag BaOmer - Segulas & Minhagim
R' Moshe ben R' Yisroel Iserles, the Rama, see Yitz's comment
R' Shlomo ben R' Moshe of Kishenev (1828) Talmid of the Rebbe, Reb Elimelech of Lizensk and the Rebbe, Reb Shmelke of Nikolsberg. He said he would help whoever mentions his name together with his mother's name, R' Shlome ben Feiga
R' Shlomo ben R' Shlomo Amar (1872) -Shetilei Zeitim
R' Moshe Kohen Narol (1659), Rov of Metz and author of Sefer Kel Molei Rachamim
R' Uri Feivel HaLevi , wrote Sefer Mikdash Me'at
19 Iyar
Maharam MiRuttenberg, R' Meir ben R' Boruch, died in prison because he wouldn't allow the Jews to pay an enormous ransom to free him (1293) Rebbe of the Rosh
R' Shmuel Abuchatzeira - The forefather of the Abuchatzeira dynasty, which includes the Abir Yaakov and Baba Sali. Origianally the family name was Elbaz, but it was changed to Abuchatzeira, which means master of the carpet, after the well know story of the him travelling on top of a carpet over the sea, after he was refused passage on a ship. He was a talmid of R' Chaim Vital, and is mentioned by the Chida in Shem Hagedolim as being a great tzadik. He was born in Eretz Yisroel but lived in Syria for a while.
R' Menachem Mendel ben R' Yosef "Charif" Thurm of Rimanov (1815) He said he would do a favor for one who lights a candle for his neshama. He was one of the 4 main talmidim of the Rebbe, Reb Elimelech of Lizhensk.
R' Ezra Attiah, Rosh Yeshivas Porat Yosef (1970)
R' Yaakov Moshe Soloveitchik of Zurich, Rosh Yeshiva in Lucerne (1995)
R' Avraham Chaim Pinaso (1896) Wrote sefer Beis El
R' Moshe of Brosholi, talmid of the Chofetz Chaim
20 Iyar
R' Saadia Marjok of Yerushalaim (1773)
R' Mordechai ben R' Menachem Nochum Twersky, Magid of Chernobyl (1838) See: Magid Of Chernobyl - Pure Simple Emunah
R' Chaim Avrohom ben R' Moshe Gagin of Constantinople (1848), Rishon Letzion & Rosh Yeshiva of Beit El, wrote Mincha Tehora on Menachos and Responsa Chukei Chaim.
R' Yitzchok Eizik Halevi Rabinowitz, wrote Doros Harishonim to counter the false history written by maskilim
R' Eliyahu Elejimi, Mekubal and Av Beis Din in Yerushalaim
R' Yechi ben R' Avrohom Abuchatzeira (1935)
R' Yosef Valtuch, mekubal in Eretz Yisroel (1983)
22 Iyar
R' Mordechai Shraga Feivish Friedman of Husiyatin (1894) youngest son of R' Yisroel of Rizhin
R' Shlomo Eliezer ben R' Yaakov Alfandri, the Saba Kadisha (1930) See here for more.
23 Iyar
R' Shalom Bashari, Dayan in Yemen (1773)
24 Iyar
R' Binyomin ben R' Menachem Mendel Mendelson, Rov of Komemiyus. He was one of the founders of Moshav Komemiut, in Eretz Yisroel, which was one of the first moshavim to keep the laws of shmita. See here for the famous story about how their crops grew better after shmitah than the ones who didn't observe shmitah. Interestingly, he dies the Monday after Behar-Bechukosai, which speaks about shmitah and the blessings that come through its observance. This is testament to the deeds of this great Rov.
R' Yaakov ben R' Yaakov Moshe Lorberbaum of Lisa (his father died before he was born, hence, the name), wrote Chaavas Daas, Nesivos and Derech Hachaim. Great-grandson of the Chacham Tzvi.
R' Eliezer Tzvi ben Yitzchak Issac Yehuda Yechiel Safrin of Komarna (1898), wrote Damesk Eliezer on Zohar
R' Chezkiah Aharon Chaim Pinkarli, Rov in Italy
R' Yitzchak Ibn Dana (1900), Rov in Fez, Morocco, wrote L'Yitzchok Reiach
R' David HaCohen Sakali (1948), wrote Kiryas Chanah Dovid
25 Iyar
R' Ozer ben R' Meir HaKohen of Klimintov (1710), Even HaOzer
R' Chaim ben R' Menachem Mendel Hager of Kosov (1854), wrote Sefer Toras Chaim, he was the father of the first Vizhnitzer Rebbe.
R' Avrohom Yosef ben R' Meir Ash, Chassidish Rabbi in New York in the second half of the 19th century. He is buried in Queens, NY, in one of the old cemeteries.
26 Iyar
Rabbeinu Sa'adia Gaon (942)
Ramchal, R' Moshe Chaim Luzzato ben R' Yaakov Chai (1747), wrote many seforim, including Mesilas Yeshorom, Derech Hashem and Daas Tevunos. Died at age 39, which the Magid of Mezerich said was because the generation was not worthy of such a great tzadik. The Vilna Gaon said he did not find one extra word in the first 8 chapters of Mesilas Yesorim and that if he were still alive he would walk from Vilna to Italy to learn from him.
R' Yitzchok ben R' Chaim of Volozhin
R' Shlomo ben R' Mordechai of Zvhil (1945) See Reb Shlomke's Miraculous Cholent
R' Shmuel Eliyahu of Zvolin, son of R' Yecheskel of Kuzmir and father of the first Modzhiter Rebbe
R' Yitzchok Issac Epstein of Homil (1857), See: Reb Aizel Homiler Does a Jew a Favor
27 Iyar
R' Avrohom Shmuel Bacharach (1615), married the grandaughter of the
Maharal of Prague and was the grandfather of the Chavas Yair
R' Yitzchok Abulafia (1825), Av Beis Din of Tzefas
R' Shlomo Zalman ben R' Yehuda Leib Schneerson of Kapust (1900), author of Mogen Avos, grandson of the Tzemach Tzedek of Lubavitch
R' Eliezer Zev of Kretchnif HY"D (1944) ben R' Meir of Kretchnif
R' Yerachmiel Yehuda Meir ben R' Shimon Sholom Kalish, the Amshinover Rebbe (1976)
28 Iyar
Shmuel Hanavi (878 B.C.E.) and his mother Chana
R' Yitzchak of Kurweil, author of the S'mak, Sefer Mitzvos Kotton (1280)
29 Iyar
R' Meir ben R' Aron Aryeh Leib of Premishlan. See here for more.
R' Shmuel Shlomo of Radzin HY"D (1942) ben R' Mordechai Yosef Elazar
