Thursday is the feast of St John the Baptist. On La Palma, they have the very sensible custom of holding the party on the night before the public holiday, so that everyone can sleep late the morning after. Since fiestas often go on until the early hours, sleeping in next morning is delicious.
In Catholic countries, people celebrate the feast of St John with bonfires. (Any similarity to pagan midsummer festivals is purely coincidental.) On Wednesday, there will be lots of small, private bonfires, plus bigger ones in: San Andrés y Sauces, San Juan Belmaco (Malpaíses in Mazo). El Remo (in Fuencaliente), and in Puerta Naos (Los Llanos), the witches will dance round a bonfire on the beach.
On Thursday, there will be special church services in Puntallana (main village), and Belmaco in Malpais, Mazo.