From "GIFTS FOR THE SOUL" - educational essays on the holidays and seasons of the Jewish year. Published by feldheim.com.
It is a mitzvah of the Torah to count the days between Passover and Shavuot -- for Shavuot marks the calendar date when HaShem spoke at Mount Sinai and gave the Jewish People the Torah. The counting of the days, which is done once a day for each of the 49 days, is called Sefirat HaOmer, "counting of the omer." An omer is a measure of grain, and when the Temple stood, an offering the size of an omer of the year's first wheat flour was brought on the altar on the holiday of Shavuot.
The period between Passover and Shavuot is called the time of the "sefirah" (counting). One counts the days in anticipation of the holiday of Shavuot. The word Shavuot itself means "weeks" -- seven weeks between Passover and Shavuot.
As a result, one might presume that the period between Passover and Shavuot is a time of joy. Actually, the case is quite the opposite. The sefirah is a period of serious introspection, for the Jewish People suffered many tragedies during the time of the sefirah. Thus it is a time for trying particularly hard to increase one's dedication to Torah and mitzvot.
Although all of Jewish history is replete with suffering, many tragic events took place during the period of the counting of the omer. Probably the most famous tragedy was the plague that killed 24,000 disciples of Rabbi Akiva. According to the Midrash (Bereshis Rabbah 61:3; Koheles Rabbah 11:10), all of Rabbi Akiva's disciples died "during the same stretch of time," and the Talmud specifies that it was between Passover and Shavuot (Yevamos 62b).
This was such a blow to the Jewish people, its effects are still being felt. The disciples of Rabbi Akiva were the pride of the nation and its hope for the future: brilliant Sages who were just starting out upon the long road to greatness in Torah. Not long before, the Second Temple had been destroyed, and the only consolation that the nation could take was that at least Torah study remained strong. When these 24,000 perished, however, the nation was plunged into a deep state of despair.
In the times of the Crusades, there were more tragedies of immense proportions during the sefirah. In 1095 CE, during the sefirah, the Jews of over one hundred Ashkenazic communities were mercilessly massacred by the first Crusade. These included the large communities of the cities of Shair, Worms, and Magenca. In Shabbat prayer services between Passover and Shavuot, special lamentations are added to the liturgy to commemorate these massacres. Also, the prayer "Av HaRachamim," which is said all year round on Shabbat to eulogize Jewish martyrs, was composed primarily as a result of the devastation caused by the first Crusade. In some communities, "Av HaRachamim" is said only during the sefirah.
Fifty years later the second Crusade set out, between Passover and Shavuot. Again, thousands of Jews were massacred. According to Polish Jewish tradition, a special hymn written about this tragedy is to be said in the very beginning of the sefirah -- on the first Shabbat after Passover.
So, too, many of the "transports" to the death camps during the Holocaust were carried out during the sefirah.
Also, every year, the public reading of Parashat Bechukosai falls during the sefirah. It is said in the name of the Vilna Gaon that this was intentional, for this Torah portion prophetically describes the terrible suffering that the Jewish people would have to endure for failing to remain faithful to the Torah. The Sages who lived long before the Crusades, and before the times of Rabbi Akiva, too, instituted the weekly readings. According to the Vilna Gaon, the Sages of old foresaw that the period between Passover and Shavuot would be filled with sadness. Therefore they intentionally scheduled Bechukosai to be read during this period in particular, to clarify to the nation what, in fact, would be the cause for its woes.
As noted, the Talmud and the Midrash relate that during the sefirah there was a tragic plague that took the lives of all of the students of Rabbi Akiva -- 24,000 disciples. On the other hand, one day during the sefirah is a day of joy, for on this day, just before he died, it is said that Rabbi Shimon Bar Yochai revealed great amounts of "hidden" wisdom -- secrets of creation that, until then, HaShem had shared only with him.
The day that Rabbi Shimon died (his yahrtzeit), the day that he revealed this treasured secret wisdom -- was the 33rd of the 49 days of the counting of the omer. Therefore, this happy day in the midst of the sad sefirah (counting) is called Lag BaOmer, the 33rd day of the omer.
Rabbi Shimon illuminated all of Israel by expounding not only the "revealed" part of the Torah, but also the Torah's "secret" wisdom about creation -- the Kabbalah -- wisdom that is nistar, or "hidden." The teachings of Rabbi Shimon about Kabbalah were considered as authoritative as the "revealed" part of the Torah which HaShem taught to Moses and which Moses passed on to the entire Jewish nation in the manner described above.
There were many great Torah scholars in his generation, but in the realm of nistar, Rabbi Shimon towered over them all.
Rabbi Shimon became a disciple of Rabbi Akiva only after the plague that killed Rabbi Akiva's 24,000 disciples... After this additional tragedy, Rabbi Akiva had to start over again to try to restore the vast amount of Torah knowledge that had been lost. He gathered to his side seven noted Torah scholars -- Rabbi Meir, Rabbi Yehudah, Rabbi Yosei, Rabbi Shimon, Rabbi Elazar Ben Shamua, Rabbi Yochanan HaSandlar and Rabbi Eliezer Ben Ya'akov -- and began to teach them. The Talmud says that five of these seven (the first five) "stood the Torah back on its feet." According to his own contemporaries, Rabbi Shimon was "the lion of the group." What is more, Rabbi Shimon was said to be "anointed with joyous oil" -- graced with knowledge of nistar -- more than his colleagues.
It appears that this yahrtzeit became a special day on the Jewish calendar because the Jewish nation had to have one holiday set aside to rejoice over the nistar part of the Torah -- over the knowledge that was the specialty of Rabbi Shimon Bar Yochai.