I happened to reach Hospital de Orbiga at the time of the festival last year, having no idea it was on. It was an amazing colourful chaotic atmosphere. the streets of the village were lined with stalls with a Medieval theme - food, blacksmithing, leather working etc. Everyone was in a really happy, friendly party mood. As well as the main ceremony by the bridge in the evening, there was also a display that was a cross between morris dancing, cheerleading and flag waving by a group of Italians(!) throwing their flags high in the air and weaving in out of each other in intricate patterns.
I stayed in the amazing old Municipal Refugio in the main street, which has a lovely old courtyard as well as a garden out the back. I think it might be a problem finding somewhere to stay during the festival. It may be worth catching a bus or taxi up from Leon or back from Astorga.
When I left the following morning, the party was still going in the next village. There was foliage in all the streets,a sound system and around 30 people in the streets who invited me to join them in a glass of wine.
Mercury