The following is from my book “Mysteries of Judaism.”
The semi-holiday Lag B’omer is based on an unsubstantiated and unlikely tradition. Lag B’omer means the thirty-third day since bringing the barley harvest (omer) to the tabernacle and later temples to thank God for the barley harvest. An omer is a certain weight of the grain such as an ounce or pound. The Torah states in Leviticus 23:15 that the sheaf of barley was brought to the tabernacle “after Shabbat” and the rabbis interpreted this as “on the day after the first day of Passover.” The thirty-third day therefore occurs on the eighteenth day of the Jewish month Iyar, which begins May 22, 2020 in the evening through May 23.
The Encyclopedia Judaica, Vol. 10, states that the day has been “celebrated as a semi-holiday since the time of the geonim.” A practice had existed of engaging in mourning during the period after Passover until the holiday of Shavuot, although different communities began and ended the grieving at different times. Jews who observed the practices during the seven weeks did not cut their hair and beards, celebrate marriages, or listen to music. The mourning practice may have begun some time before the Talmudic period but, as previously mentioned, was modified during the gaonic period when Lag B’omer began to be observed.
Midrash Genesis Rabbah 61:3, edited around 400 CE, reports: “Rabbi Akiva [who died around 135 CE] had twelve thousand disciples, from Gabbath to Antipatris, and all of them died during the same time . Why [did they die]? Because they differed with each other.” The Babylonian Talmud, Yevamot 62b, edited around 600 CE, tells the story somewhat differently. “Rabbi Akiva had twelve thousand pairs of disciples, from Gabbathto Antipatris, and all of them died during the same time because they didn’t treat each other with respect.” Neither source mentions the mourning practices. Because the Talmud was copied in the early days by hand, there are some minor variations in the different copies. One copy of the Talmud adds that they died during the same period “until close to Shavuot.” Jews interpreted this addition as signifying the deaths stopped on the thirty-third day after Passover, Lag B’omer, the thirty third day after the omer was brought.
There is no plausible reason for selecting the thirty-third day rather than any other day since the text states “until close to Shavuot,” but doesn’t give a date. However, some rabbis supposed that the manna began to fall in the desert during the days of Moses on this day. This may account for the selection of this day; the rabbis liked to say that several events occurred on each special day. Additionally, Kabbalists imagine that the second century Rabbi Simeon bar Yohai, who they say composed the thirteenth century CE book Zohar, died on this day, and these mystics usually celebrate the anniversary of deaths as a joyful occasion. It is possible, but unlikely, that the tradition of bar Yohai’s death existed before Lag B’omer was selected as the day the disciples ceased dying.
Rabbi Akiva was convinced that Bar Kokhba, the leader of the revolt against Rome during the war in Israel between 132 and 135, was the messiah who would defeat Rome who, under the generalship of Pompey in 63 BCE, annexed Judea to Rome. Rabbi Akiva was certain that Bar Kokhba would restore Jewish sovereignty to Israel and was very vocal about his support of Bar Kokhba.
Scholars believe that when the Midrash and Talmud talk about disciples they are referring to the men who enlisted into Bar Kokhba’s army because of this “teaching” of Rabbi Akiva. Ancient wars were frequently fought in the spring after the rainy season, when the ground was firm, after Passover.
Bar Kokhba lost the war and thousands of his soldiers died. This is what was meant by the death of twelve or twenty four thousand students. Bar Kokhba’s final battle was at the fortified city Beitar and many scholars believe that some Judeans informed the Romans how they could breach the fortifications. This is hinted in the words “because they differed with each other” and “because they didn’t treat each other with respect.”
Why, then is Lag B’omer celebrated as a semi-holiday? It is a custom
developed by Jews based upon the defeat of the Judeans by Rome in 135 and the
cessation of hostilities, but virtually all Jews forgot the real reason.
Instead, they substituted a myth to explain its origin, a day when scholars
stopped dying from a plague.
 The term “semi-holiday” means that the day has certain religious practices but is not a day in which work is prohibited.
 I will discuss the reason for this unusual interpretation in another essay.
 Babylonian religious leaders from around 600 to 1038.
 Ashkenazic Jews, those of Germanic and western European origin, do not engage in the mourning practices on Lag B’omer. Jews of Sephardic descent, from Spanish and Arab countries, do not stop the mourning on the 33rd day but on the 34th day. The mourning customs vary among Ashkenazic Jews. Some observe the restrictions for the entire seven weeks, with the exception of Lag B’omer. Some do not start the practice until after the month of Nissan in which Passover is observed is over. Some end the practice on the 33rd day and do not observe it for the 34th to 49th day. Others just cease observing it on the 33rd and resume mourning on the 34th.
 Meaning, from northern to southern Israel, the whole country.
 As I pointed out, there are usually different versions of laws and events in different midrashic texts even when the rabbis are addressing the same law and event.
 For example, they say that about a half dozen significant events occurred on the ninth day of Av, including the destruction of the First and Second Temple. Actually neither temple was destroyed on that day. This notion of multiple events occurring on the same day seems to be the superstitious notion that there are good days and bad days, days on which to start activities and days on which to avoid starting them, such as the 9th of Av.
 Including Rabbi Akiva.
 There are other rational reasons for rejecting the myth. It is unlikely that God becomes involved in murdering scholars even if they are bad; we do not see this occurring at other times. It is also unlikely that only scholars did not respect other scholars; many people do not respect their neighbors and many do not like the opinions of every scholar. We have no evidence that there was a plague and even if there was a plague, why did it only affect scholars? Furthermore, there were many rabbis at that time. Why were only the students of Rabbi Akiva disrespectful and why were only they punished?