That contrast is part of the point.
Lag BaOmer is the 33rd day of the Omer count
Britannica defines Lag BaOmer as the minor Jewish observance on the 33rd day of counting the Omer, the period between Passover and Shavuot.
The name itself comes from that count.
It interrupts a semi-mourning period
Britannica explains that the Omer period is treated as a time of semi-mourning and that on Lag BaOmer the restrictions ease, allowing weddings and haircuts.
That is what gives the day its odd energy. It is relief inside restraint.
Bonfires and pilgrimage shape the holiday
Britannica notes traditions such as bonfires, celebrations, and pilgrimage to Meron in connection with Rabbi Shimon bar Yochai.
These customs make the day feel far more public and exuberant than many minor observances.
Why it still matters
Lag BaOmer still matters because Jewish time is not flat. Even within a season of waiting and restraint, tradition can preserve a sanctioned eruption of joy.
The shortest accurate answer
Lag BaOmer is the minor Jewish observance on the 33rd day of the Omer count, marked by a break in mourning customs and by festive celebration.