The period of mourning (for the 24,000 students of Rabbi Akiva who died of plague) associated with Sefirat Ha’omer ends on the 33rd day of the Omer, a day known as Lag Ba’omer. In Hebrew, every letter has a numerical value. "Lamed" equals 30, and "Gimmel" equals 3, thus Lag (spelled Lamed Gimmel) Ba'omer, literally means 33 (days) in the Omer.
Because the mourning period is now over, Lag Ba’omer is a popular date for weddings (which are not held during Sefirat Ha’omer) and haircuts. Many have the custom not to cut a boy's hair until he is three years old, the age at which the child first begins to learn Torah. Since haircuts are delayed until after the period of mourning, and because there is Kabbalistic significance to hair, many put off the hair-cutting ceremony, called an Upsherin, until Lag B'Omer.
Lag Ba’Omer is also the anniversary of the death of Rabbi Shimon Bar Yochai, the famed Kabbalist who revealed his teachings in the Zohar. In Israel, tens of thousands of people travel to Mount Meron (near Safed) to observe his yahrtzeit near the cave in which he was buried. Per his deathbed request, his death is celebrated, rather than mourned.
It is also common for families and friends to gather together for a bonfire and/or picnic on Lag B'Omer, often on Mount Meron. There are several reasons given for this custom. One is that Rabbi Shimon Bar Yochai’s Zohar translates to “shining light,” and bonfires bring light to the world.