Hundreds of thousands of Israelis throughout the country began celebrating the mystic holiday of Lag Ba’Omer on Saturday night with giant bonfires, barbecues, prayers and parties.

The minor holiday, which marks the death of famed Kabbalist Rabbi Shimon Bar Yochai, takes place on the 33rd day of the Omer, a seven-week period religious Jews count between Passover and the holiday of Shavuot.

By far the largest celebration occurs at the tomb of Rabbi Bar Yochai on Mount Meron in Israel’s north.

Tens of thousands of Ultra-Orthodox Jews traditionally make a pilgrimage to the tomb for Lag Ba’Omer, where they dance and pray around dozens of enormous bonfires. The celebration is a feat of logistics, including hundreds of special buses from all over the country, distribution of over 550,000 liters of water, tight security and medical and fire services.

The origins of the holiday are somewhat mired in mystery. According to the Talmud, the day marks the end of a divinely-sent plague which killed 24,000 students of the major 2nd century sage Rabbi Akiva for the sin of treating each other without proper respect.

After the plague, Rabbi Akiva was left with only five disciples, among them Rabbi bar Yochai, who went on to become the greatest Torah scholar in his generation. He is thought to be the author of the Zohar, the foundational text of Kabbalah, or Jewish mysticism.

According to the Zohar, Rabbi bar Yochai revealed the deepest secrets of Kabbalah on the day of his death, making Lag Ba’Omer into a celebration of light coming into the world.

Today, that mystical light is represented by giant bonfires, which Israeli children excitedly prepare days in advance, scavenging their neighborhoods for spare wood and flammable materials. The day is one of great joy and celebration.