Lag BaOmer is a Jewish festival celebrating the anniversary of the passing of Rabbi Shimon Bar Yochai, who was the first to publicly teach the Kabbalah which is the mystical dimension of the Torah. He is the author of the Zohar. Before he died he asked his disciples to celebrate the date as “the day of my joy.” It is believed that the day of the death of a righteous person’s life on earth marks the culmination of their deeds, teachings and the height of the impact on the world. So each year they celebrate this life and the revelation of the esoteric soul of Torah.

Lag BaOmer is also commemorated because it is written in the Talmud that in the 49 days between Passover and Shavuot (another Jewish celebration) a plague raged through the disciples of Rabbi Shimon teacher Rabbi Akiva because they did not act respectfully towards each other. On Lag BaOmer the dying ended so it carries is imortalises the theme of respecting each other.

Further information

For further information on how Lag BaOmer is celebrated and why children traditionally play with bows and arrows see here. There is also a mystical reference to 33.  https://www.chabad.org/library/article_cdo/aid/679300/jewish/What-Is-Lag-BaOmer.htm