Reb Ovadyah of Bartenura zt'l writes in a letter (written in Eretz Yisrael and sent to Italy), "On the eighteenth of Iyar, the day of his [Reb Shimon's] yahrtzeit, people from all around Meron go to Meron and light bonfires... Many barren bear children, the ill are healed, because of the vows they take on at that place."
In this letter, Reb Ovadyah of Bartenura mentions that the miracles come from the vows that people make in Meron. The vows refer to the good kabbalos (such as giving tzedakah or taking on a good practice) that people accept on themselves when they are in Meron, on Lag b'Omer. It is sufficient to accept the kabbalah on oneself bli neder, without an actual vow.
Also, when Reb Shimon lived, he brought many salvations.
Earthquake Miracles
It is written in the sefer מירון מסע that in Cheshvan 1761/ב"תקכ, there was an earthquake in Tzefas and Meron, and many Arabs, with their wives and children, ran to the building over Reb Shimon’s grave for protection, but the door was locked. They shouted, "Reb Shimon! Reb Shimon! We heard about your greatness from our fathers and witnessed it with our eyes. Open your doors." Miraculously, the doors opened, and they went inside. Reb Shimon became even more revered by them at that time.
A month later, on the 6th of Kislev, there was a second earthquake. Yidden and lehavdil goyim ran to Reb Shimon’s grave. They found the building jumping up and down. The dome on the roof split in two. They became terrified. They shouted, "Reb Shimon! Reb Shimon! If this is happening to your building, what will be with us?" Finally, after half an hour, the earthquake settled, and the building sat firmly in place. They went inside to inspect the damage, but there was none. The dome merged, and there was no sign of any scratches or damage.
Reb Shimon Stops a Plague
The Zohar (Rus 104) says: There was an epidemic in Lud. People took Reb Shimon around the city so he could see their suffering with his own eyes. Reb Shimon said, "How can it be that I’m in this city, and there’s a plague? I decree that the plague should stop!" A bas kol came forth and said to the מלאכי חבלה (damaging angels), "You must leave this city. Reb Shimon bar Yochai is here. Hakadosh Baruch Hu decrees in heaven, and Reb Shimon annuls the decrees on earth."
Reb Shimon Saves the World
In another location, the Zohar says that the world suddenly became dark. Reb Shimon bar Yochai told his son, Reb Elazar, "Come with me, and we will see what Hakadosh Baruch Hu is planning to do to the world." They found a malach, tall like a high mountain, with thirty flames of fire coming out of its mouth. Reb Shimon asked the malach what it was planning to do. The malach replied, "It states (Bereishis 18:18) ואברהם יהיה היו, and יהיה is gematriya thirty. This means that in every generation, there must be at least thirty tzaddikim similar to Avraham Avinu. But now, there aren't thirty tzaddikim in the world like Avraham, so Hashem sent me to destroy the world."
Reb Shimon bar Yochai said to the malach, "Go to Hakadosh Baruch Hu and tell Him that Yochai's son is in the world." The malach went to Hakadosh Baruch Hu and said, "Ribono Shel Olam, You know what Reb Shimon bar Yochai told me to tell You." Hakadosh Baruch Hu said, "Go and destroy the entire world, and don’t pay attention to what Yochai's son says."
The angel returned, ready to fulfill the mission Hashem sent him to do. Reb Shimon bar Yochai saw the malach and said, "If you don't leave, I will decree that you will never return to your place in heaven. I will send you to a forsaken place. Therefore, listen to what I tell you. Go to Hakadosh Baruch Hu and say, 'Even if there aren't thirty tzaddikim in the world, twenty tzaddikim are also sufficient to save the world, as Hashem told Avraham (Bereishis 18:31), העשרים בעבור אשחית לא, 'I will not destroy on account of the twenty [tzaddikim].' And even if there are only ten tzaddikim, that is also enough to protect the world from destruction because Hashem said (Bereishis 18:32), העשרה בעבור אשחית לא, 'I will not destroy on account of the ten.' And even if there are only two tzaddikim in the world – Reb Elazar and me – it is also sufficient... And if there is only one tzaddik in the world, it is also enough, as it states, עולם יסוד צדיק, a tzaddik is the foundation of the world."
At that moment, a bas kol came forth from heaven and said, "Reb Shimon, fortunate is your lot. Hakadosh Baruch Hu places His decrees in heaven, and you annul them from the earth. About you it is written (Tehillim 145:19), יעשה יראיו רצון, 'The will of those who fear Him, He will do.'"
