On June 23rd Spaniards celebrate the day of St. John (San Juan in Spanish), the roots of which go back to paganism. It is believed that after this longest day of the year is over, Light begins to lose its strength and Darkness takes over. To support Light, people of San Sebastian start making bonfires after the sunset. They burn everything old and unneeded - from letters and documents to furniture. One can also write down anything that s/he wants to get rid of and throw it into the fire. On the night of June 23, the bonfires are burning in the city squares and even on Zurriola beach, but everything begins at the Constitution Square at 20:00. 

There a tree is traditionally burned and the locals dance Basque folklore dances around it. Anyone can join this celebration.

More details here