A few quotes from the Rabbi on 'Customs of Israel'
in which he teaches that the day of Rabbi Shimon Bar Yochai may be likened to the Giving of the Torah itself
In 1915, during the World War, may a disaster not occur twice, we were exiled from the Holy City to Na Ammon [Egypt] and, on Lag Ba’Omer, I was asked to say a few words of Torah…
This holy and awesome day, marking the decease of the greatest of our masters, Rabbi Shimon Bar Yochai, may his acclaim protect us Amen, has become a festival day of joy and celebration throughout the world’s Jewish communities. It is a very puzzling matter, since on the anniversary of the decease of the greatest of our masters, Rabbi Shimon Bar Yochai it would seem appropriate that we fast, weep and mourn at our loss of the world’s light. Yet we do the opposite, strangely, feasting in joy and celebration… How odd!
However, one part of the Torah, a superior part, was not revealed to the world until the day of Rabbi Shimon Bar Yochai’s decease, may he rest in peace, when permission was granted for its revelation. So that the day of Rabbi Shimon Bar Yochai’s decease is like the day of the Giving of the Torah…and since it was revealed on that day, this superior part could now exist for generations to come. This day, therefore, has become one of feasting and joy, and a festival likened to the day of the Giving of the Torah itself.
Am Mordecai, Sermon for Lag Ba’Omer, p. 35b, HaMaarav Press, Jerusalem, 1933