What is a Yom Hillula

Image via Wikipedia//Meron on Lag BaOmer

A Yom Hillula (Hebrew: יום הילולא‎) is another word for yahrzeit (the anniversary of someone's passing), but it refers to a great Tzaddik. Unlike a regular yahrzeit, marked with sadness and even fasting, a Yom Hillula is commemorated specifically through simcha, and festive celebration. This term is most often used in Hasidic circles to refer to the day of the passing of Hasidic Rebbes. Initially, was mostly used for festivities, but according to a late interpretation (Moses Alsheikh) the death of a saintly man is a kind of "mystical marriage" of his soul with G-d.

The first Tzaddik whose Yom Hillula was celebrated is that of Rabbi Shimon bar Yochai, on Lag BaOmer. Rabbi Shimon Bar Yochai referred to this day, whose celebration was a specific request to his students, as Yom Simchato ("the day of his happiness"). There is thus a very widely observed custom to visit Meron (the burial place of Rabbi Shimon bar Yochai) on Lag BaOmer, and celebrate with torches, song, and feasting. Another celebration takes place on the 14th of Iyyar, the anniversary of the death of R. Meir Ba'al ha-Nes, at their respective gravesides in Meron and Tiberias, in Galilee. In galut, this anniversary was observed with great solemnity in the Jewish community of Djerba (Tunisia). It consisted of a procession with a richly ornamented candlestick (menorat Shimon bar Yoḥai), which was followed by eating and drinking with musical accompaniment. A similar hillula was observed in Libya and Morocco (and now in Israel) on the first day after Passover, called Maimuna in honor of the anniversary of the death of Maimonides, who died on Passover.

Sources:

http://www.jewishvirtuallibrary.org/jsource/judaica/ejud_0002_0009_0_08968.html