Lag Ba'omer is a Jewish holiday which takes place on the 18th day of the month of Iyar in the Jewish calendar. So in 2007, the holiday of Lag Ba'Omer takes place on Sunday May 6, beginning at sundown on the previous day, Saturday May 5.
The name Lag Ba'Omer refers to the thirty-third day of the counting of the "Omer," a holiday season that lasts for a total of 49 days, starting from the second day of Passover. The word "Lag" is not really a word; it is a number. In Hebrew, as in ancient Greek, numbers were written with letters: lamed, ל, L = 30, and gimel, ג, G = 3, so L-G ("Lag") is 33.
The Lag Ba'Omer holiday goes back to the time of Rabbi Akiva. According to the Talmud, twenty-four thousand of the rabbi's students died from a plague that had been sent upon them as a punishment for not showing respect to each other. The plague came to an end on the 33rd day of the counting of the Omer, which is commemorated in the Lag Ba'Omer holiday.
According to other accounts, the holiday really commemorates the many soldiers who died in the Bar Kochva revolt which took place during Rabbi Akiva's lifetime.
The holiday is celebrated with dancing, singing and picnics. Bonfires are lighted, and children play games with bows and arrows. The day is a school holiday, and "Students' Day" is celebrated at many universities. You can read more about Lag Ba'Omer at About.Com or at the Velveteen Rabbi blog.
Here's a Lag Ba'Omer poster showing the different symbols of the holiday, available from Avron Judaica: