It is observed in the Ashkenazic community by permitting haircuts and shaves, music and wedding celebrations, all traditionally omitted during the light mourning period in memory of the demise of the Rabbinical students of Rabbi Akivah. In Israel, it is marked by bonfires and a massive commemoration.
Since Rabbi Akivah regarded there as being a correlation between sin and punishment, the reason they attribute for the plague is the hateful divisions between his students.
Historians have a different take on this period.
They regard it as a period of intense loss in the military battles between the Bar Kokhba rebels and the Romans in the revolt of the years 132-135, which resulted in the defeat of the rebels and the exile of the Jews from Israel.
It is not the only time when Jewish history and traditional Jewish memory clash.
Yet even historians who debunk the traditional interpretation of Lag Ba'Omer might advocate continuing, if not strengthening, the tradition of light mourning for in modern Jewish history these week coincide with the deportation and annihilation of Hungarian Jews in 1944.
Recall Hungarian Jews faced persecution, discrimination even forced labor but not systematic murder until the Germans invaded on March 19, 1944. Jews were then ghettoized in April often beginning with Passover, and between the 15th of May and the 8th of July some 437,402 Jews were deported primarily to Birkenau [Auschwitz II] where 8 of 10 were murdered upon arrival, 54 days, 147 trains, no respite on the 33rd of the Omer.
So, one might also want to consider this period of light mourning as a remembrance of the Jews of Hungary. Its most famous, Elie Wiesel for example, observed Passover in his home with his parents and sisters, the Wiesel family was intact. On the 7th day of Passover, the day in the synagogue that Jews read the Moses’ Song from the Sea, the Wiesel family was ghettoized. The arrived in Auschwitz on Shavuot itself, the very day his mother and his young sister Tzipporah were sent to the gas chambers of Birkenau.
The reason the Talmudic sages attributed to plague are of unique importance at this moment in time when Jews who have returned from exile to the ancestral home of Israel are so deeply divided that even as we celebrated the 75th anniversary of Israel’s founding and the magnificent accomplishments of this monumental period in Jewish history, we had less confidence in the nature of Israel’s future than perhaps anytime since the Yom Kippur War when it seemed as if the Third Commonwealth might be destroyed from without. The most urgent question of this Lag Ba'Omer: will Israel be destroyed by divisions within?