(1) It is customary not to get married between Pesach and Shavuot, until Lag BaOmer (the 33rd day), because during that time, the students of Rabbi Akiva died. However, to do "erusin" and "kiddushin" (engagement and betrothal) is OK. And even for "nisuin" (marriage), if someone did so, we do not punish him. Rema: however, from Lag Ba'Omer onwards, all this is permitted (Abudraham, Beit Yosef & Minhagim).
(2) It is customary not to cut one's hair until Lag BaOmer, since it is said that that is when they stopped dying.

(15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to the Lord.

Bountiful are You, Eternal One our God, Sovereign of the Universe, who has made us holy with commandments and commanded us regarding the counting of the Omer.

Today is....days, which makes .... days and...weeks of the Omer.

(18) Rabbi Eliezer son of Hisma said: the laws of mixed bird offerings and the key to the calculations of menstruation days these, these are the body of the halakhah. The calculation of the equinoxes and gematria are the desserts of wisdom.

Rabbi Akiva says that the verse should be understood as follows: If one studied Torah in his youth he should study more Torah in his old age; if he had students in his youth he should have additional students in his old age, as it is stated: “In the morning sow your seed, etc.”They said by way of example that Rabbi Akiva had twelve thousand pairs of students in an area of land that stretched from Gevat to Antipatris in Judea, and they all died in one period of time, because they did not treat each other with respect.And the world was desolate of Torah until Rabbi Akiva came to our Rabbis in the South and taught his Torah to them. This second group of disciples consisted of Rabbi Meir, Rabbi Yehuda, Rabbi Yosei, Rabbi Shimon bar Yochai, and Rabbi Elazar ben Shamua. And these are the very ones who upheld the study of Torah at that time. Although Rabbi Akiva’s earlier students did not survive, his later disciples were able to transmit the Torah to future generations.

With regard to the twelve thousand pairs of Rabbi Akiva’s students, the Gemara adds: It is taught that all of them died in the period from Passover until Shavuot. Rav Ḥama bar Abba said, and some say it was Rabbi Ḥiyya bar Avin: They all died a bad death. The Gemara inquires: What is it that is called a bad death? Rav Naḥman said: Diphtheria.

The Sages taught: Askara (diphtheria) comes to the world as punishment for neglecting to separate food meant for tithes. Rabbi Elazar, son of Rabbi Yosei, says:Askara comes as punishment for slander.

[Rabbi Akiva said to his new students:] the first students died because their eyes were narrow in the Torah they shared with each other. Do not be like this. They immediately stood up and filled the land of Israel with Torah.

Rebbi Simeon ben Yohai stated: Akiba my teacher used to preach, there appeared a star out of Jacob there appeared Koziba out of Jacob. When Rebbi Akiba saw Bar Koziba he said, “this is King Messiah”. Rebbi Yochanan ben Torta said to him, “Akiba! Grass will grow from your jaws and still David’s son (still has to) [will not have] come.”

R' Akiva gave himself over to execution after R' Yosi ben Kisma died. R' Chanina ben Tradiyon was also killed and wisdom diminished after they passed. R' Akiva raised up many students, but a religious persecution waged against his students...

In this baraita Rabbi Yehuda is described as head of the speakers in every place. The Gemara asks: And why did they call him head of the speakers in every place? The Gemara relates that this resulted due to an incident that took place when Rabbi Yehuda and Rabbi Yosei and Rabbi Shimon were sitting, and Yehuda, son of converts, sat beside them. Rabbi Yehuda opened and said: How pleasant are the actions of this nation, the Romans, as they established marketplaces, established bridges, and established bathhouses. Rabbi Yosei was silent. Rabbi Shimon ben Yoḥai responded and said: Everything that they established, they established only for their own purposes. They established marketplaces, to place prostitutes in them; bathhouses, to pamper themselves; and bridges, to collect taxes from all who pass over them. Yehuda, son of converts, went and related their statements to his household, and those statements continued to spread until they were heard by the monarchy. They ruled and said: Yehuda, who elevated the Roman regime, shall be elevated and appointed as head of the Sages, the head of the speakers in every place. Yosei, who remained silent, shall be exiled from his home in Judea as punishment, and sent to the city of Tzippori in the Galilee. And Shimon, who denounced the government, shall be killed. Rabbi Shimon bar Yoḥai and his son, Rabbi Elazar, went and hid in the study hall. Every day Rabbi Shimon’s wife would bring them bread and a jug of water and they would eat. When the decree intensified, Rabbi Shimon said to his son: Women are easily impressionable and, therefore, there is room for concern lest the authorities torture her and she reveal our whereabouts. They went and they hid in a cave. A miracle occurred and a carob tree was created for them as well as a spring of water. They would remove their clothes and sit covered in sand up to their necks. They would study Torah all day in that manner. At the time of prayer, they would dress, cover themselves, and pray, and they would again remove their clothes afterward so that they would not become tattered. They sat in the cave for twelve years. Elijah the Prophet came and stood at the entrance to the cave and said: Who will inform bar Yoḥai that the emperor died and his decree has been abrogated?They emerged from the cave, and saw people who were plowing and sowing. Rabbi Shimon bar Yoḥai said: These people abandon eternal life of Torah study and engage in temporal life for their own sustenance. The Gemara relates that every place that Rabbi Shimon and his son Rabbi Elazar directed their eyes was immediately burned. A Divine Voice emerged and said to them: Did you emerge from the cave in order to destroy My world? Return to your cave. They again went and sat there for twelve months. They said: The judgment of the wicked in Gehenna lasts for twelve months. Surely their sin was atoned in that time. A Divine Voice emerged and said to them: Emerge from your cave. They emerged. Everywhere that Rabbi Elazar would strike, Rabbi Shimon would heal. Rabbi Shimon said to Rabbi Elazar: My son, you and I suffice for the entire world, as the two of us are engaged in the proper study of Torah. As the sun was setting on Shabbat eve, they saw an elderly man who was holding two bundles of myrtle branches and running at twilight. They said to him: Why do you have these? He said to them: In honor of Shabbat. They said to him: And let one suffice. He answered them: One is corresponding to: “Remember the Shabbat day, to keep it holy” (Exodus 20:8), and one is corresponding to: “Observe the Shabbat day, to keep it holy” (Deuteronomy 5:12). Rabbi Shimon said to his son: See how beloved the mitzvot are to Israel. Their minds were put at ease and they were no longer as upset that people were not engaged in Torah study.

The Sages taught: What is the meaning of that which the verse states: “And you shall gather your grain”? Because it is stated: “This Torah shall not depart from your mouths, and you shall contemplate in it day and night” (Joshua 1:8), I might have thought that these matters are to be understood as they are written; one is to literally spend his days immersed exclusively in Torah study. Therefore, the verse states: “And you shall gather your grain, your wine and your oil,” assume in their regard, the way of the world; set aside time not only for Torah, but also for work. This is the statement of Rabbi Yishmael.Rabbi Shimon ben Yoḥai says: Is it possible that a person plows in the plowing season and sows in the sowing season and harvests in the harvest season and threshes in the threshing season and winnows in the windy season, as grain is separated from the chaff by means of the wind, and is constantly busy; what will become of Torah? Rather, one must dedicate himself exclusively to Torah at the expense of other endeavors; as when Israel performs God’s will, their work is performed by others, as it is stated: “And strangers will stand and feed your flocks, and foreigners will be your plowmen and your vinedressers” (Isaiah 61:5). When Israel does not perform God’s will, their work is performed by them themselves, as it is stated: “And you shall gather your grain.” Moreover, if Israel fails to perform God’s will, others’ work will be performed by them, as it is stated: “You shall serve your enemy whom God shall send against you, in hunger, in thirst, in nakedness and in want of all things” (Deuteronomy 28:48). Summing up this dispute, Abaye said: Although there is room for both opinions, many have acted in accordance with the opinion of Rabbi Yishmael, and combined working for a living and learning Torah, and although they engaged in activities other than the study of Torah, were successful in their Torah study. Many have acted in accordance with the opinion of Rabbi Shimon ben Yoḥai and were not successful in their Torah study. They were ultimately forced to abandon their Torah study altogether.

Rabbi Shimon said: Since a miracle transpired for me, I will go and repair something for the sake of others in gratitude for God’s kindness, as it is written: “And Jacob came whole to the city of Shechem, which is in the land of Canaan, when he came from Paddan-aram; and he graced the countenance of the city” (Genesis 33:18). Rav said, the meaning of: And Jacob came whole, is: Whole in his body, whole in his money, whole in his Torah. And what did he do? And he graced the countenance of the city; he performed gracious acts to benefit the city. Rav said: Jacob established a currency for them. And Shmuel said: He established marketplaces for them. And Rabbi Yoḥanan said: He established bathhouses for them. In any event, clearly one for whom a miracle transpires should perform an act of kindness for his neighbors as a sign of gratitude. He said: Is there something that needs repair? They said to him: There is a place where there is uncertainty with regard to ritual impurityand the priests are troubled by being forced to circumvent it, as it is prohibited for them to become ritually impure from contact with a corpse. There was suspicion, but no certainty, that a corpse was buried there. Therefore, they were unable to definitively determine its status. Rabbi Shimon said: Is there a person who knows that there was a presumption of ritual purity here? Is there anyone who remembers a time when this place was not considered ritually impure, or that at least part of it was considered to be ritually pure? An Elder said to him: Here ben Zakkai planted and cut the teruma of lupines. In this marketplace Rabbi Yoḥanan ben Zakkai, who himself was a priest, once planted lupines that were given to him as teruma. On that basis, the conclusion can be drawn that it was definitely ritually pure. Rabbi Shimon, like Jacob, also did so and took steps to improve the city and examined the ground (Tosafot). Everywhere that the ground was hard, he pronounced it ritually pure as there was certainly no corpse there, and every place that the ground was soft, he marked it indicating that perhaps a corpse was buried there. In that way, he purified the marketplace so that even priests could walk through it. A certain Elder said in ridicule and surprise: Ben Yoḥai purified the cemetery. Rabbi Shimon got angry and said to him: Had you not been with us, and even had you been with us and were not counted with us in rendering this ruling, what you say is fine. You could have said that you were unaware of my intention or that you did not agree or participate in this decision. Now that you were with us and were counted with us in rendering this ruling, you will cause people to say that Sages are unwilling to cooperate with one another. They will say: If competing prostitutes still apply makeup to each other to help one another look beautiful, all the more so that Torah scholars should cooperate with each other. He directed his eyes toward him and the Elder died. Rabbi Shimon went out to the marketplace and he saw Yehuda, son of converts,who was the cause of this entire incident. Rabbi Shimon, said: This one still has a place in the world? He directed his eyes toward him and turned him into a pile of bones.

Truculent stingers and shield-bearing warriors from Sepphoris came and beset them. The people of Meron banded together and shouted, for they feared he would not be buried there. After the bed emerged from the house, it rose into the air; fire blazed before it. They heard a voice: “Come and gather! Assemble for the celebration of Rabbi Shim'on!"
Zohar: The Book of Enlightenment, Translated by Daniel Matt, p. 188

God further said, “This is the sign that I set for the covenant between Me and you, and every living creature with you, for all ages to come.

לדרת עולם FOR GENERATIONS FOR EVER — The word לדרת “for generations” is written missing a letter vav (implying that the sign will be necessary only for such generations as are “missing’’ in faith) because there will be some generations which will require no sign since they were completely righteous, such as the generation of Hezekiah, king of Judah, and the generation of R. Simeon ben Yochai.

Rabbi Chizkiyah said in the name of Rabbi Yirmiyah: During the lifetime of Rabbi Shimon bar Yochai, a rainbow was never seen in the clouds.

One particular custom practiced on the day of Lag B'Omer is unique: Children go to parks and fields to play with bows and arrows. What is the reason for this peculiar custom? One well-known explanation has to do with the fact that during Rabbi Shimon's lifetime, no rainbow ever appeared in the sky. This is profoundly significant, because Genesis relates that the rainbow represented G-d's covenant never to destroy the world again even if the human race would degenerate to its status prior to the Flood. But as long as Rabbi Shimon was alive, his merit and piety alone were enough to ensure that G-d would not regret His creation, with no need for the rainbow. On the day of Rabbi Shimon's passing, however, the world was in need of the rainbow. Thus, each year on that day we recall this man's greatness by playing with the bow.

(5) You shall keep My laws and My rules, by the pursuit of which man shall live: I am the LORD.

§ The Gemara now considers which prohibitions are permitted in times of mortal danger. Rabbi Yoḥanan says in the name of Rabbi Shimon ben Yehotzadak: The Sages who discussed this issue counted the votes of those assembled and concluded in the upper story of the house of Nitza in the city of Lod: With regard to all other transgressions in the Torah, if a person is told: Transgress this prohibition and you will not be killed, he may transgress that prohibition and not be killed, because the preserving of his own life overrides all of the Torah’s prohibitions. This is the halakha concerning all prohibitions except for those of idol worship, forbidden sexual relations, and bloodshed. Concerning those prohibitions, one must allow himself to be killed rather than transgress them.