Behind this ritual there is an ancient story that tell about an "evil" father, who does not believe in God, who constantly tries to kill his own son, instead believer. This guy also had a blessed sister that apparently was invulnerable to the fire. His sister proposes to save the boy sitting with him in the midst of the fire but something goes wrong and the girl burns. But Somehow he survived managing to escape.
So, practically the ritual consists in saving the boy (the trunk) from the holy fire (his sister). The people then gather trying to extinguish the fire and burn a few sticks to take home for later blessing. In the big cities there are more fires around the streets, instead in the small villages is lit up only one huge fire where all people gather.
P.S. The story and the ritual was quite complicated to understand and explain, I hope you got more or less the story.