The Talmud (Yevamot 62b) relates that “Rabbi Akiva had 12,000 pairs of students...and all of them died in one period of time because they did not act with respect toward each other...they all died between Passover and Shavuot...and they all died a terrible death.” The Talmud then goes on to explain that, according to Rabbi Nachman, the deaths were caused by a croup-like illness that resulted in the victim’s suffocation.
Think of it: 24,000 students! And all but 5 of them (as stated elsewhere in the Talmud) succumbed to this horrible plague.
Unfortunately, love of God and love of God’s Torah, does not always translate into proper moral comportment. Instead of encouraging each other’s pursuits in learning, the students busied themselves with showing off their own Torah knowledge in order to “one-up” their fellows. As punishment for this great failing, the students were struck by the plague.
As the Talmud notes, this great tragedy occurred during the time period between Passover and Shavuot and lasted until Lag Ba’omer, the 33rd day of the Omer. For this reason, 33 days of Sefirat Ha’omer (the Counting of the Omer) are considered days of mourning.
Communal mourning in Jewish tradition is expressed by the Jewish people by refraining from certain activities. During the 33 days of Sefirah, the precluded activities include: 1) cutting hair, 2) going to live performances of musical entertainment and 3) getting married.