Lag Ba'Omer is a festive day on the Jewish calendar, celebrating the anniversary of the passing of the great sage and mystic Rabbi Shimon bar Yochai, author of the Zohar. It also commemorates another event. In the weeks between Passover and Shavuot, a plague raged amongst the disciples of the great sage Rabbi Akiva. On Lag BaOmer the dying ceased.
Lag Baomer this year is on May 10, 2012.