The 8th of August is San Ciriaco, the patron Saint of Ibiza Town. It is also known as the day the Virgin, to honour Nuestra Señora de las Nieves (Our Lady of the Snows).
As every year, there will be a fantastic firework display from Ibiza's port at midnight.
Before that, there is also a public afternoon snack for free in Puig des Molins (Google maps).
You can find more information about this traditonal fiesta in our article Ibiza Fiestas - Patron Saints' Days of Nuestra Senora de las Nieves and San Ciriaco.