From Judaism 101:
In memory of a plague during the lifetime of Rabbi Akiba. Haircuts during this time are also forbidden. The 33rd day of the Omer (the eighteenth of Iyar) is a minor holiday commemorating a break in the plague. The holiday is known as Lag b'Omer. The mourning practices of the omer period are lifted on that date. The word "Lag" is not really a word; it is the number 33 in Hebrew, as if you were to call the Fourth of July "Iv July" (IV being 4 in Roman numerals). See Hebrew Alphabet for more information about using letters as numbers.As restrictions of mourning are lifted on this 33rd day of the Omer, weddings, parties, listening to music, and haircuts are commonly scheduled to coincide with this day. Families go on picnics and outings. Children go out to the fields with their teachers with bows and (rubber-tipped) arrows. Tachanun, the prayer for special Divine mercy on one's behalf is not said, because when God is showing one a "smiling face," so to speak, as He does especially on the holidays, there is no need to ask for special mercy.
There was at one time a dispute as to when the counting should begin. The Pharisees believed that G-d gave Moses an oral Torah along with the written Torah, and according to that oral Torah the word "Shabbat" in Lev. 23:15 referred to the first day of Passover, which is a "Shabbat" in the sense that no work is permitted on the day (Rosh Hashanah and Yom Kippur are both referred to as "Shabbat" in this sense, though they cannot both occur on a Saturday in the same year; see Lev. 23:24 and 23:32; see also Lev. 23:39 the first and eighth days of Sukkot are called "Shabbat"). In this view, held by most Jews today, the counting begins on the second night of Passover, that is, the day after the non-working day of Passover. The Tzedukim (Sadducees) rejected the idea of an oral Torah and believed that the word "Shabbat" in Lev. 23:15 referred to the Shabbat of the week when Pesach began, so counting would always begin on a Saturday night during Passover. The Sadducees no longer exist; today, only a small sect call the Karaites follow this view.
In Meron, the burial place of Rabbi Shimon bar Yochai and his son, Rabbi Elazar, hundreds of thousands of Jews gather to celebrate with bonfires, torches, song and feasting. This was a specific request by Rabbi Shimon bar Yochai of his students.
In Israel, Lag BaOmer is a school holiday. Youngsters and their parents light bonfires in open spaces in cities and towns throughout the country. Students' Day is celebrated on the campuses of the various universities. Lag BaOmer is also a favorite day for weddings.
In Israel, one knows that Lag BaOmer is drawing near when children begin collecting wood boards, The most well-known custom of Lag BaOmer is the lighting of bonfires. 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.
An important significance to the the bonfires is that they represent signal fires that the Bar Kochva rebels lit on the mountaintops to relay messages.
The Bnei Yissaschar cites another reason for the lighting of bonfires. On the day of his death Rabbi Shimon Bar Yochai said, "Now it is my desire to reveal secrets...The day will not go to its place like any other, for this entire day stands within my domain..." Daylight was miraculously extended until Rabbi Shimon had completed his final teaching and died. This symbolized that all light is subservient to spiritual light, and particularly to the primeval light contained within the mystical teachings of the Torah. As such, the custom of lighting fires symbolizes this revelation of powerful light.
At the tomb of Rabbi Shimon, the honor of lighting the main bonfire traditionally goes to the Rebbes of the Boyaner dynasty. This fire is lit on the roof of the tomb at 2:00 a.m.
On this day of celebration, have fun. Take the family out for a picnic. Get a haircut. Dance joyously. Burn Obama in effigy. The last one will make you feel 100% better.