Lag B’omer is not a well known or celebrated holiday, and it’s not Biblical in any sense.
It is a holiday that commemorates the stoppage of a plague that afflicted the students of the famous Rabbi Akiva of the first century. According to tradition, they suffered a plague as punishment for not treating each other with the proper respect. In the words of the sages, they related to each other with Sinat Hinam/baseless hatred, allowing their competitive natures to get the better of them and treated each other miserably.
The plague ended on Lag B’omer (33rd day of the Omer, corresponding to 18 Iyar), and today we celebrate by focusing on kindness and love and on how we should treat each other to build healthy societies.
The holiday has taken on new customs throughout the ages, with building raging bonfires and having carnivals and parades being the most prominent. In Israel, the day is also reserved for taking school children on wilderness adventures and introducing them to the outdoors.