Q. What is the source of people lighting bonfires on Lag Baomer?
A. Sefer Ranu LeYaakov (p. 214) mentions echoing Chassam Soffer, that the source for this very widespread tradition, is the pasuk “Eish Das Lamo’ (Devarim 33: 2) that well describes the constant fiery fight and combat the Rabi Shimon Bar Yochai led against those who opposed the learning of Torah.
See also Lebush Yosef (p. 215), Ginas Reuben (p.13), Vezos LeYehuda (p. 229) and others that quote the minhag and offer other reasons. Many of them are based on the symbolic representation of fire to the learning and teaching of Torah, especially of the Nitzotzos or sparks of Kedusha inherent in the Zohar and Cabalah teachings that were released on that day
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Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a