Lag B’Omer is a very important holiday because, first of all, it symbolizes the end of the correction of the vessels. On the whole there are seven Sefirot: Hesed, Gevura, Tifferet, Netzach, Hod, Yesod, and Malchut (HGT NHYM), and each one includes the same seven Sefirot.
The 33rd Sefira is Hod of Hod, which is like Malchut of the first nine Sefirot, where the upper bestowal ends. If we perform the correction until Hod of Hod, then the correction will certainly take place in the remaining Sefirot because the work of correction is actually over and then it is only put into practice from Yesod of Hod through the general Yesod and the general Malchut.
So Lag B’Omer is celebrated as a special day in the correction of the common soul; the work is already completed. Afterward, everything will undoubtedly be fulfilled correctly.
Therefore, it is customary to light bonfires, because it is the holiday of light. It is also clear that from this day onward, Rabbi Shimon was not obliged to remain with his students because he finished his work, and therefore he left this world.
We continue to count and clarify the days of the Omer to the end, until Shavuot, but, from the top down, all the Sefirot are already filled with Light and the whole state is ready. Therefore, it is the day the great illumination called Rashbi departs.
From the 1st part of the Daily Kabbalah Lesson 5/18/14, The Zohar