Dancing, singing, and a day when bonfires are lit and children traditionally play with bow and arrows on a day of rejoicing in the middle of a period of mourning.
In the weeks between Pesach and Shavuot, a plague raged amongst the disciples of Rabbi Akiva “because they did not act respectfully towards each other”; these weeks of the Omer are observed as a period of mourning, with joyous activities proscribed. On Lag B’Omer the dying ceased. Therefore haircuts, listening to music, holding weddings etc. are permitted on this day.
Also the anniversary of the death of Rabbi Shimon bar Yochai. On the day Rabbi Shimon passed away, a great light of endless joy filed the day, because of the secret wisdom he revealed to his students. This wisdom was written down and recorded in the Zohar and because of the joy and happiness back then, we celebrate with happiness now, as well.