With roots in the pagan celebration of the summer solstice, the Festividad de San Juan [Feast of St. John] celebrates the birth of St. John the Baptist while retaining a number of pre-Christian rituals. The Feast of St. John falls on June 24th, but it's not unusual for the festivities to take place on an alternate date sometime around the solstice. 
Midsummer festivals and celebrations in honor of St. John are not unique to one particular corner of the world; however, I'll focus on the traditions of Spain's Basque region (País Vasco), since those are the customs that have influenced Necochea's Festividad de San Juan.
In the heart of the País Vasco, on the eve of the Feast of St. John (San Juan Sua), bonfires are lit to ward off evil spirits. Custom dictates that the young people must jump over the fire three times to purify themselves and bring good luck. It's also traditional to throw objects into the fire that represent things the person desires to change or leave in the past (e.g. a smoker may throw a pack of cigarettes in the fire if he wishes to quit). Sometimes these wishes are written on scraps of paper that are tossed into the fire. 
A tremendous wave of European immigration at the end of the 19th century/beginning of the 20th century brought a number of Basques to Argentina. The immigrants kept alive many of their traditions from the "old country," and so the Festividad de San Juan continues here in Necochea, which is home to a significant Basque community.
This year's Festividad de San Juan was held on June 20th in front of the Centro Vasco Euzko Extea (the Basque Cultural Center) in Necochea. In contrast to the celebrations in the northern hemisphere, the festivities here took place on the eve of the winter solstice. A giant papier–mâché witch was mounted above a bundle of sticks in preparation for a roaring bonfire. Merrymakers launched firecrackers and then set the towering witch ablaze as costumed dancers twirled about, illuminated by the fire. Following the burning of the witch, a huge dinner was served in the Centro Vasco (I know a good witch burning always makes me hungry).
Here's hoping your summer/winter solstice (whichever the case may be) was free of evil spirits. I know mine was.