Shimon is regarded as the author of the mystical work, the Zohar.
His Yahrzeit is known as Hillula (“Festivity”) d’Rabbi Shimon bar Yochai and is a great event at Meron, in the Upper Galilee near Tz’fat (Safed), where the rabbi is buried. Thousands of people gather there to sing and dance and to celebrate the first haircut of three-year old boys.
Why a death anniversary can be a celebration is explained on the basis that every year on the Yahrzeit the soul of a deceased tzaddik is elevated to a higher rung.
Another Lag Ba’Omer custom associated with Rabbi Shimon is that of children playing with bows and arrows. “Bow” is keshet, which is also a rainbow, which in the story of Noah is a symbol of peace (Gen. 9:11-17).
It is said that no rainbow appeared while Rabbi Shimon was alive. His righteousness was itself a guarantee that peace would come.
Some link Rabbi Shimon with Lag Ba’Omer on the basis that, living in the time of the Romans and a leading opponent of their policies, he is a symbol of the independent Jewish spirit which motivated the Bar Kochba revolt of which Rabbi Akiva was the spiritual mentor.
On this basis the bows and arrows represent Jewish defiance.