This evening will be the solstice, when the sun appears at its farthest north in the sky. For the northern hemisphere, it’s the longest day of the year. (And for the southern hemisphere, it’s the longest night of the year.)
La Palma has several archaeological sites which mark the solstice. One of the easiest to get to is El Verde in El Paso. Park beside the cemetery and follow the footpath. It’s not far, but it is rough underfoot. You want to get there by about 7:30 pm-
El Verde is a natural amphitheatre, with a notch in the western wall. On the solstice, the sun sets in this notch and the last bit of sunlight illuminates one of the rock carvings. See http://www.youtube.com/watch?v=0_2emQJIpPA&list=HL1339768534&feature=mh_lolz (I think embedding is disabled)
At San Antonio volcano visitor centre in Fuencaliente there’s a modern solstice marker which looks like a mini Stonehenge. At sunset on the solstice, the shadow of one stone touches another.