Tonight is Lag B’Omer. It is generally understood that this is the day when the plague that killed the 24,000 students of רבי עקיבא had ended.
The lesser known reason for celebrating Lag B’Omer is that this is the Yahrtzeit of רבי שמעון בר יוחאי. It is also referred to as the Hilula of רבי שמעון.
It was Rabbi Shimon, himself, who requested that the anniversary of his death be celebrated with rejoicing and light, rather than sadness and mourning. Tradition relates that its sun had not set until he had revealed all that he had been permitted to reveal. It was Rashbi that had written the Zohar when he hid in the cave for twelve years.
The bonfires lit are looked as massive Yahrtzeit candles celebrating the life of Rabbi Shimon Bar Yochai.