Q. Kevod Harav. Why is the Yohrzait of Moshe Rabbenu, the Seventh of Adar considered a day of mourning and is kept as the day dedicated for the Chevra Kadisha, while Lag Baomer which is the day we commemorate the Yohrzait of Rabi Shimon Bar Yochai is a day of great joy and Simcha?
A. The reason is likely to the Talmud (Temurah 16a) teaching that Rav Yehuda said in the name of Shmuel: Three thousand Halochos were forgotten during the days of mourning for Moshe Rabbenu. Yet the day when Rabi Shimon Bar Yochai passed away is described by Shaar HaKavonos by Rabbi Chaim Vital and others, as Yom Simchoso, since he gave on that day the greatest spiritual light to the world with the revelation of many of the Zohar secrets.
That turned it into a Yom Hillula, a day of celebration, and bonfires and lights are lit to symbolize the impact of his teachings
Horav Shlomo Miller’s Shlit’a agrees to this reason.
Other reasons are mentioned including the fact that Moshe Rabenu passed away when he was in distress, as he so much desired to enter the Land of Yisrael and was denied.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a