Yahrtzeit of Rabbi Shimon Bar Yochai (100-170 CE), the great scholar and leader during the Roman period following the destruction of the Second Temple. After criticizing the Roman government, Rabbi Shimon was forced to go into hiding with his son for 13 years. They lived in a cave, studying Torah, and subsisting on carob and water. Rabbi Shimon is credited with authoring many works of Jewish law, including the Zohar, the primary source of Jewish mysticism, which he revealed to his students on his final day. Lag B'Omer -- literally the "33rd day of the Omer" -- is the yahrtzeit of Rabbi Shimon and, according to his wishes, is observed as a day of great celebration. Jews throughout Israel light huge bonfires, symbolically illuminating the deeper truths of Torah, as revealed by Rabbi Shimon. The main celebration is at Meiron, the Rabbi Shimon's burial site, where tens of thousands of people gather to light torches, sing and dance in his honor.
Iyar 18 is also the yahrtzeit of Rabbi Moshe Isserles (1530-1572), known by the acronym Rema. His most famous work was notes on the Code of Jewish Law (Shulchan Aruch); these opinions are regarded as the definitive source for Ashkenazi Jewry. The Rema was born and died in Krakow, Poland, where he served as rabbi of the city. Not only a Talmudic and legal scholar, he was also learned in Kabbalah, and studied history, astronomy and philosophy. He penned a Torah scroll that was preserved for centuries in his synagogue in Krakow, until being destroyed in the Holocaust.