Chazal (Bava Kama 9) say שליש עד מצוה הידור, that a person should add a third onto the mitzvos.
As the Gemara (Succah 52:) states, "The yetzer hara battles a person every day and seeks to kill him. If Hakadosh Baruch Hu wouldn’t help, he wouldn’t overcome the yetzer hara." Hashem helps him by completing the final third of the battle.
The Maharsha (Moed Katan 28. מת ה"ד) explains that the fifty days of Sefiras Ha'Omer represents the fifty years of a person's life after he completes his first twenty juvenile years. Each day of Sefiras Ha'Omer represents another year of those fifty years. On Lag b'Omer, two-thirds of the Omer has passed, symbolizing that most years of a person’s life have passed. Chazal tell us that when one lives most of his life without sin, he won't sin anymore. The Maharsha writes, "We celebrate a minor holiday on Lag b'Omer because, on this day, two-thirds of the Omer passed, which represents that most days of his life has passed, and when most of one's life passes without sin, he won't sin for the remainder of his life."
Rashi writes that Yosem ben Uziyahu "Was a tzaddik, more humble than other kings, and excelled in kibud av. About him, it states (Malachi 1:6), אב יכבד בן, 'A son honors his father.' All the days his father had tzaraas, and Yosem was judging the nation, he didn’t place the crown on his head. Instead, he attributed all his judgments to his father."
After counting the omer, we say (Tehillim 67), בנגינות למנצח. This chapter has 49 words (aside from the first pasuk). It is ideal to focus on the word of this chapter that corresponds to each night (as is taught by the mekubalim and printed in many siddurim). On Lag b'Omer, the focus is on the word אלקים. This hints that on Lag b'Omer, even Elokim, Hashem's name for strict judgment, agrees to redeem the Jewish nation from the judgment. אלקים is roshei teivos for דין ה ןמ עולםה תא פטורל ניא כלי, "I can redeem the entire world from judgment."
Reb Moshe Leib Shachor of Yerushalayim zt'l told a story about a doctor from Konigsberg, Germany, who worked day and night to heal all the patients affected by an epidemic. When everyone recovered, the doctor went to a neighbouring city to rest. He arrived at a hotel and instructed the owner not to tell anyone he was there—he needed to rest. But then, the epidemic broke out in the city where he was staying, but the doctor remained concealed. He didn't want to reveal himself because he would need to work hard again to heal everyone. It is obvious that the doctor acted incorrectly. He had the ability to heal, so he should have used his knowledge to heal.
Reb Moshe Leib Shachar concluded, "Reb Shimon announced that he could save people from judgment, so we can be assured he will do so. It wouldn't be proper to have this strength and not use it.
The Sar Shalom of Belz zt'l asked that Hashem keeps the entire Torah (see Brachos 6). How does He keep the concept of adding one-third onto the mitzvos? The answer is a Yid does 2/3rds of the fight against the yetzer hara, and Hashem completes the final one-third of the battle. The Sar Shalom added that Sefiras Ha’omer is 49 days, and on the 33rd day, two-thirds of the omer has passed. During the first two-thirds of the omer, we counted the omer, and we strove to purify ourselves. Then, on Lag b'Omer, Hashem completes the mission, and we become entirely cleansed from all sin.
Rebbe Shalom Kaminka zt'l and the Divrei Chaim of Sanz zt'l were present at the Sar Shalom's Lag b'Omer tish when the Sar Shalom taught this lesson, and for the next forty-five minutes, the Sanzer Rav cradled his head in his hands, and Rebbe Shalom of Kaminka cried copiously as they were thinking about this vort with immense hislahavus.
The Gemara (Succah 45:) states that Reb Shimon bar Yochai said, "I can free all people from judgment [so they won't be punished for their aveiros]. If I add the merits of my son, Reb Elazar, we can redeem people from judgment from the beginning of Creation until today. And if we add the merits of Yosam ben Uziyahu, we can redeem everyone from judgment from the day the world was created until the end of time."
