LAG B'OMER: THE COUNTING OF THE OMER begins tonight at sundown. Best wishes to all observing it.
The biblical background of the holiday is Leviticus 23:15, which mandates the counting of seven weeks between Passover and Shavuot (Weeks). Lag B'Omer just means the 33rd day of this count (the 18th of Iyar). The festival commemorates, among other things, the traditional date of the death of Rabbi Shimon bar Yochai (Yohai).
Last year's Lag B'Omer post is here with links, and see also here and here. More recent posts are here, here, here, and here, with links.