Shimon bar Yochai

They said that Rabbi Akiva had twelve thousand pairs of students in an area of land that stretched from Gevat to Antipatris in Judea, and they all died in one period of time, because they did not treat each other with respect. And the world was desolate of Torah until Rabbi Akiva came to our Rabbis in the South and taught his Torah to them. This second group of disciples consisted of Rabbi Meir, Rabbi Yehuda, Rabbi Yosei, Rabbi Shimon, and Rabbi Elazar ben Shamua. And these are the very ones who upheld the study of Torah at that time. Regarding the twelve thousand pairs of students, it is taught that all of them died in the period from Passover until Shavuot.

(1) Mechaber: It is customary not to get married between Pesach and Shavuot, until Lag BaOmer (the 33rd day), because during that time, the students of Rabbi Akiva died...

Rema: however, from Lag Ba'Omer onwards, all this is permitted (Abudraham, Beit Yosef & Minhagim).

(2) It is customary not to cut one's hair until Lag BaOmer, since it is said that that is when they stopped dying...

(1) R’ Shimon bar Yochai was called “the holy lamp” because through him were revealed the secrets of Torah, that is the secret of “the light that was good” which is hidden in the Torah. Therefore his holy book is called the Zohar (lit. brilliance), meaning a light which shines from one end of the world to the other – the ‘good light’ which is hidden away in the Torah. That is why this day is the best of days for the giving of Torah. This is when the good light in the Torah began to shone forth, as we have learned. And this day is the 33rd day of the Omer, Lag Baomer. This is the day of the hilulah (lit. rejoicing) of R’ Shimon bar Yochai, the very day on which he ascended to the highest heavens. It also makes sense to assume that he was born on this day as well, because the Holy One sits and completes the years of the righteous from day to day (Kiddushin 38a). The very day on which ‘the light that is good’ began to shine out from the Torah, that is the best of days before the giving of the Torah, is the very day on which the holy soul that would reveal the path of ‘the light that is good’ in Torah was itself revealed. That is why this holy soul was called “the holy lamp,” on the very day that he rose up to the highest heavens he commanded R’ Aba to write down all the storehouses of hidden knowledge which he had revealed in order to leave behind a blessing from the illumination of the hidden light (that is good, as it says “How abundant is the good that You have in store for those who fear You…” (Psalms 31:20)) It is preserved there for us and our children until the light of our righteous Messiah is revealed, speedily and in our day! It is as our Sages said “And God said, let there be light…” this refers to the light of the King Messiah (Pesikta Rabbati 36), in the merit of our occupation with ‘the light that is good’ which the holy lamp left behind as a blessing and life, and a light for all Israel.