Although not a full-fledged festival and it is not mentioned in the Torah, Lag Ba'Omer is a day with much historical significance and a time of celebration in an otherwise bleak period in the religious calendar.
The seven weeks from Pesach to Shavuos are counted a day at a time, tracing the days from the Exodus to its climax, when the Ten Commandments were given at Mount Sinai. Lag B'Omer, literally the "33rd day in the counting of the Omer," was just another day until two unrelated events took place. Rabbi Akiva, who lived nearly 19 centuries ago, was one of the greatest sages in a golden era of Torah scholarship. He had an astounding total of 24,000 students, and almost all of them died between Pesach and Shavuos. The reason? They failed to show proper respect for one another. On Lag Ba'Omer the plague stopped.
One of Rabbi Akiva's few surviving students was the great Rabbi Shimon Bar Yochai, author the mystical classic, the Holy Zohar. He died on Lag Ba'Omer, but before he passed away, he revealed a dazzling array of Kabbalistic teachings to his students. He told them that they should almost celebrate the day when so many mystical secrets were revealed and taught!
The weeks between Pesach and Shavuos are a somber period, because of Rabbi Akiva's tragedy and because the Crusaders massacred countless Jews during these days. But Lag Ba'Omer is the exception. It remains a day of joy and celebration.