Lag Ba'omer is a holiday that traces back not to antiquity, but to the Middle Ages, this year it is on Sunday May 18. Marked on the 18th day of the Jewish month of Iyar, Lag BaOmer has another significance based on the Kabbalistic custom of assigning a Sefirah to each day and week of the Omer count. As such, Lag BaOmer represents the level of spiritual manifestation or Hod that would precede the more physical manifestation of the 49th day (Malkhut she-be-Malkhut, Kingship within [the week of] Kingship), which immediately precedes the holiday of Shavuot.
The most well known custom of Lag BaOmer is the lighting of bonfires throughout Israel and worldwide. The fire symbolizes the light that is wisdom, spread by the great Rabbi bar Yochai. In Meron, the burial place of Rabbi Shimon bar Yochai and his son, Rabbi Eleazar, hundreds of thousands of Jews gather throughout the night and day to celebrate with bonfires, torches, songs and feasts, this was a specific request by Rabbi Shimon bar Yochai's students. Some say that as bar Yochai gave spiritual light to the world with the revelation of the Zohar, bonfires are lit to symbolize the impact of his teachings. As his passing left such a "light" behind, many candles and/or bonfires are lit.
Festivities include making a bonfire and roasting potatoes, franks, marshmallows and other fire-friendly foods on the flames.