He authored the holy Zohar, a landmark text of Jewish mysticism / Kabbalah.
On the day of his Hillula, he revealed the deepest secrets of the Kabbalah.
Why we go into the field:
On this day, it is customary for young Yeshivah students, to leave the four walls of their holy studies, and go out into to the mundane fields.
Rabbi Shimon was so great, that he could unite the deepest mystical elements of the Torah, with the limited, physical aspects of the world.
In emulation of him, children extend their holy studies in Yeshivah to include even the external field, an area seemingly beyond the realm of Torah.
Rabbi Shimon taught, that the synthesis and underlying unity of Torah and the mundane worldliness, should be expressed every day, not only once a year.
In the Era of the Redemption, the fusion between the material and the spiritual exemplified by Rabbi Shimon, will be spread throughout the world.
And through spreading the unlimited, powerful teachings of Rabbi Shimon, the coming of the Redemption can be hastened.
Every year in the Rasbi’s resting place in Meiron, Israel (near Tzfas) over 500,000 people from all walks of life, gather together to celebrate this wondrous day.
There is a source in the Kabbalah, that Moses was reincarnated as Rabbi Shimon Bar Yochai, in order to give this mystical level of the Torah, to the Jewish people.
The most well-known custom of Lag Ba'Omer, is the lighting of bonfires. As The Rashbi gave spiritual light to the world, with the revelation of the Zohar, so too bonfires are lit, to give physical revelation of the powerful light, of his teachings.
In the words of the Zohar, lie the secrets; whose seed will bring about the coming of Moshiach.
The Chassidic masters explain, that the final day of a righteous person's earthly life, marks the point at which "all his deeds, teachings and work", achieve their culminating perfection; and the zenith of their impact upon the world.
As the Tzadik is then unemcumbered by his limited, physical body. This allows the true greatness of his soul, to freely shine its blinding light.
So each Lag BaOmer we celebrate Rabbi Shimon's life, and the powerful revelation of the esoteric soul of Torah.
He revealed more Kabbalistic secrets on that day, than he did throughout his life.
On Lag b’Omer, the immense illumination that Rabbi Shimon bar Yochai revealed, is powerfully present. It is therefore a day, when we have an extra capacity, to tap into the inner dimension of things, the soul, G-d, Torah.
Bow and arrows
One reason that some have the custom, to take bow and arrows into the field is: Rabbi Shimon's saintliness was so great, that in his merit there was no rainbow (a negative portent) during his lifetime. A rainbow is the same shape as a bow.
Another explanation is that: in order to shoot an arrow, one has to first pull back the bow. This also resembles the descent of the exile, the seeming going backwards from the high level of the time of the Beis Hamidosh.
But in reality, the descent is in order, to spring forward, like the snap of the bow, in order to propel the world forward, into the times of the Geula!
Rabbi Shimon declared, that he had seen the most elevated souls (bnei aliyah), and that they were very few. He testified, that he and his son Elazar, were two of them; and if there was only one - it was he.
He declared, that he had the power to exempt the entire world from strict judgment, from the day it was created, until his days.
The Rashbi's soul, derived from the soul of Moshe Rabbeinu, and from the soul of Moshiach.
Since the Zohar was physically introduced into the world on Lag B'Omer, its great light shines on that day. And every succeeding year, the light increases in intensity.