Lag B'omer, the 33rd day of the Omer, has a number of meanings, among them the Yom Hilula (day of death) of Rebbe Shim'on Bar Yochai, the main protagonist of the Zohar, and the day many children get their first haircut. Lag B'Omer is Hod within Hod, according to the count of the Sefirot. It is the day perhaps when the potential wating to be revealed on Shavuot is sufficiently manifest on a spirit level to allow us to breath a sigh of relief, a moment when we can plunge forward into the tachlis of manifesting the divine presence in reality.
According to one interpretation, Hod sheb'Hod represents a kind of inner Malkhut(*), while according to another interpretation, the 33rd day represents the point at which the wheat crop is mature enough to be harvested (in the way that a baby might be mature enough after 8 months of gestation to live outside the womb).
There are huge crowds that make their way on Lag B'Omer to Mt. Meron, where the tomb of Rebbe Shimon is said to be located, to celebrate Rabbi Shim'on. It's supposed to be a cave, but like many of the tombsites in Israel, it's become a generic Orthodox all-male synagogue. (From what I have read, it's supposed to be both the cave where Rabbi Shim'on is buried and the cave where he hid from the Romans, but a friend of mine says he went to the cave where Rabbi Shimon hid ten years ago, and from his description it's not the same place as the tomb. Can anyone provide information, or photos?) Regardless, the 33rd day is a celebration, a time when weddings can take place and hair can be cut after a hiatus of some weeks, and so Meron on Lag B'Omer has become the place and time to give boys their first haircut after they turn three.
The day of Rabbi Shim'on's death, the Idra Rabba, is one of the most incredible scenes in the whole Zohar, truly trippy and astonishing. According to the Zohar Rabbi Shim'on had to die after revealing such lofty secrets, but he had come into the world to reveal those very secrets.
(Does any other religious tradition have this idea that revealing secrets can cause one to die? It is also found in the idea that no one can pronounce the four-letter name of God, and that the High Priest had a rope tied around his ankle in case he died on Yom Kippur while saying the name in the Holy of Holies, and had to be pulled out.)
I know two songs related to the Sefirot that are appropriate for Lag B'Omer. One song, called Bar Yochai, is a fairly well-known song about Rebbe Shimon, which has one verse for each Sefirah of the Tree of Life starting with Malkhut. The other is El Mis'tater, "God of Secret Mystery", an old world zemer that goes through the Sefirot of the Tree of Life starting with Keter. Both songs are also sometimes sung on Shabbat; El Mistater is a seudah shlishit (Shabbat afternoon) song. You can find text and audio for both HERE.
(*) I'm not sure why Hod is imagined to be an inner Malkhut rather than Netsach, since Netsach is the seventh Sefira from Keter.