And from the day on which you bring the sheaf (omer) of elevation offering the day after the sabbath you shall count off seven weeks. They must be complete: you must count until the day after the seventh week fifty days; then you shall bring an offering of new grain to the Lord.
-Leviticus 23: 15-16
Though it might not be obvious from the biblical text, the sages interpreted the word sabbath in this passage as referring to the first day of Pesach; i.e., holy day. Thus, we start counting the omer on the second day of Pesach, and counting until we reach the fiftieth day, when we celebrate Shavuot. This was the period of the barley harvest, that began at Pesach and ended with the start of the wheat harvest. That this period assumed the character of a time of mourning has been explained in different ways: a) often in agricultural societies, at the times we are most vulnerable to natural disaster, we try not to act too happy, so as not to tempt fate. Stormy weather can destroy a ripening field of grain, so during these final weeks of the development of the grain crop, it behooves us to keep a low profile. b) according to the Talmud (Yebamot 62b) 24,000 students of Rabbi Akiba died in a plague between Pesach and Shavuot, and for them we mourn each year. When we are in mourning, we refrain from musical entertainment, and from shaving and cutting our hair, and most importantly, from celebrating marriages.
Medieval halachic manuals refer to the 33rd day of the counting of the Omer (Lag Bomer: the numerical value of the letters L and G is 33) as a day of relief from mourning. Why? The answer is not clear. Some say that the plague on Rabbi Akibas students stopped that day though there is no mention of this in the Talmud. Others say that Rabbi Shimon bar Yochai, reputed author of the Zohar (the classic work of Kabbalah Jewish mysticism), died on that day and revealed his mystical knowledge to his students on his death bed. Most people dont know either explanation only that the day is one of marriages, bonfires, and bows and arrows.
Why bonfires? Well, if you believe that Rabbi Akibas students were not actually felled by a plague, but in the fighting of the Bar Kochba revolt against the Romans, then the bows and arrows and bonfires represent the Jews hiding in the woods to study Torah (which had been forbidden), pretending to be camping and hunting if the Romans came snooping. But if you believe that the day is basically Rabbi Shimon bar Yochais yahrzeit, then the bows represent the rainbow of Gods promise, and the fires represent the great light of his death-bed revelation.
What is certain is that if you are a building contractor in Israel, you know you must guard your construction sites closely starting about a week before Lag Bomer, or the neighborhood kids will strip it of any lumber to feed their traditionally sanctioned pyromaniacal orgy, which leaves a pall of smoke over the country for the next day (and cancels all leaves for the fire department). And if you run a catering hall, this is the biggest night of the year; it is not uncommon to be invited to a couple of weddings on this day.
I flew from the US to Israel on the day before Lag Bomer this year, and I was one of the few non-Hassidim on my flight: Hassidim study the Zohar and revere Rabbi Shimon bar Yochai, and are eager to participate in the huge all-night festival of fire and dancing and eating and drinking and praying that takes place each year at his tomb in the tiny Galilee village of Meiron; this year the crowd was estimated at about 300,000 (6% of the Jewish population of the country).
Whenever you are tempted to boast about what a rational religion Judaism is, just remember Lag Bomer.