Lag ba'Omer, a minor festival that falls on the 18th of Iyyar, is the 33rd day of the seven-week counting of the Omer which takes places between Passover and Shavuot (Lag is an acronym for the Hebrew letters lamed and gimmel, whose numerical equivalent is 33). On Lag ba'Omer the semi-mourning of the Omer period is lifted, and weddings, haircuts and celebrations are permitted.

The reason for the institution of Lag ba'Omer is found in an obscure Talmudic passage (Yevamot 62b), which describes a plague that killed 24,000 disciples of 1st cent. Rabbi Akiva during the Omer period; a complicated commentary on the passage suggests that the plague ceased at the middle of the counting period, hence the Scholars' Festival on Lag Ba'Omer.

According to one more historical theory concerning the origins of the minor festival, the Jewish rebel Bar Kokhba may have secured a victory against the Romans on the 33rd day of the Counting, after a series of defeats. Roman rule made it necessary to hide the real reason for celebration and to attribute it other reasons (such as the reason mentioned above). Other scholars suggests that the taking up of arms at the outbreak of the first revolt against Rome took place on this date in 66 CE.

According to other traditions, the great Flood commenced on this date, and in the time of Moses, manna began to fall from heaven. A later tradition established La ba'Omer as the date of death R. Shimon bar Yohai (2nd cent., student of Rabbi Akiva),to whom the Zohar is attributed, and hence came to be celebrated in particular by the kabbalists. At Rabbi Shimon bar Yohai's burial place, in Meron in the Galilee, mystics and Hasidim gather on Lag ba'Omer, giving their young sons their first haircuts, lighting bonfires, and dancing and singing through the night.

Lighting bonfires, a central form of celebrating this day in Israel among religious and non-religious Jews, is quite likely unrelated to Rabbi Shimon bar Yohai but rather based on an ancient, heathen, light-ceremonial. The custom of playing with bows in arrows on Lag ba'Omer may have originated in Europe, where children customarily played in the fields, re-enacting the Jewish-Roman war.