This is a local fiesta in Santa Cruz and the Breñas, in celebration of the Sacred Cross. As often happens, the main party is the night before the public holiday. The wayside crosses will be beautifully decorated. Since most crosses are hung with gold jewelry (among other things) people stay beside them in shifts all night, and all the next day. Most holidaymakers go and admire the crosses on the morning of the 3rd (Sunday), but the locals go see the crosses starting at about 11 pm the night before, when it’s cooler and more atmospheric.
For the last few years, it’s been fairly common to have a few mayos or machangos beside the cross. These are giant rag dolls, something like scarecrows or the guys I used to make for bonfire night. Last year, one street in Santa Cruz was full of them.
If you’ve got a hire car, the best plan is to go up to San Isidro and follow the crowd down the hill. There are crosses all the way along a very steep lane, which used to be a donkey track. Tonight it’ll be one-way — downhill. (You go up the much newer, asphalted road.) As you approach each cross, you’ll find a small traffic jam. You go past the cross slowly (usually saying, “Oh wow!”) and drive on to the next.
If you haven’t got a car, I recommend a stroll around Santa Cruz, particularly up the older bits.