Lag B'Omer

(15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to the Lord.

It was said, That Rabbi Akiva had twelve thousand pairs of disciples, from Gabbat to Antifaras; and they all died at the same time because they did not treat each other with respect. The world remained desolate until Rabbi Akiva came to our Masters in the South and taught the Torah to them. and it was they who revived the Torah at that time. These were Rabbi Meir, Rabbi Judah, Rabbi Yossi, Rabbi Shimon and Rabbi Eleazar ben Shammua; And it was they who revived the Torah at that time. A Tanna taught: All of them died between Passover and Pentecost. Rabbi Hama bar Abba or, it might be said, Rabbi Chiyya bar Abin said: All of them died a cruel death. What was it? — Rabbi Nahman replied: Croup.

(1) 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. However, to do "erusin" and "kiddushin" (engagement and betrothal) is OK. And even for "nisuin" (marriage), if someone did so, we do not punish him. 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.

It has been told: On the day that Rabbi Shim'on was to depart the world, while he was arranging his affairs, the Comrades assembled at his house. Present were Rabbi El'azar, his son, Rabbi Abba, and the other Comrades. The house was full.

Zohar: The Book of Enlightenment, Translated by Daniel Matt, p. 182

Truculent stingers and shield-bearing warriors from Sepphoris came and beset them. The people of Meron banded together and shouted, for they feared he would not be buried there. After the bed emerged from the house, it rose into the air; fire blazed before it. They heard a voice: “Come and enter! Assemble for the wedding celebration of Rabbi Shim'on!"

Zohar: The Book of Enlightenment, Translated by Daniel Matt, p. 188