Every one of the forty-nine days of the Omer is characterized by a different combination of Hashem’s Attributes; Lag BaOmer’s combination is “Hod Shebehod” (“Eminence within Eminence”). The Vilna Gaon (Sifra D’Tzniuta) writes that the Attribute of Hod is composed mostly of the quality of Divine Judgement and partly of Mercy. Rabbi Shimon Bar Yohai said that the merit of him and his son Rabbi Elazar were able to protect the entire world from Hashem’s harsh judgement. In other words, they had this ability because they had reached the ideal state of man that Hashem had intended when He created the world. It is for this reason that the song “Bar Yohai” says “Na’ase Adam Ne’emar BaAvurecha” (lit. “[the words from Parashat Bereshit] ‘Let us make man’ were said regarding you”).
It is a common custom to recite Mizmor 67 (“Lamnatzeah”) after counting theOmer. After the opening verse, there are forty-nine words, each corresponding to the days of the Omer. The word from the Mizmor that corresponds to Lag BaOmeris “Elokim”, the Name of Hashem which signifies Divine Judgement. After the Flood, Hashem made of covenant with Noah, memorialized by a rainbow, that even if the world were to sin He would not destroy the world again. When Rabbi Shimon was alive, a rainbow was never seen, because his own merit was enough to assuage Hashem’s Divine Judgement.
The Gemara (Gittin 67a) relates that the corpus of Oral Torah as we know it was received and then disseminated by Rabbi Akiva, but after his students died, only Rabbi Shimon Bar Yohai was able to take over its continuity. As such, Rabbi Akiva revealed to him not only the standard oral Torah but the inner secrets of the Torah as well. Furthermore, Rabbi Shimon testified that he received all the best character traits of his master Rabbi Akiva. There are thirty-two days leading up toLag BaOmer, which has a numerical value of Lev (“heart”), and represents the transmission of Torah up until the time of Rabbi Shimon. From Lag BaOmer toShavuot, there are seventeen days, which has a value of Tov (“good”) and represents the era ushered in by Rabbi Shimon’s revelation of the secrets of the Torah. Together, these words come together to form the best of quality traits as mentioned in Pirke Avot (2:10), Lev Tov, a good heart.
Summary: Lag BaOmer celebrates the merit of Rabbi Shimon Bar Yohai and the continuity of the Oral Torah.