In reply to Solsbury:
Here is a 3 day trek. Apologies, in Spanish. https://fernandozamoraguiadepicos.com/es/senderismo-y-trekking-en-picos-de-...
It should give you a good idea.
September 8th is the local patron Saint in Asturias so should be plenty of fiestas that day.
Llanes celebrates La Guia on 6th-9th of September.
Most fiestas are July and early August though or in Oviedo from 16th-23rd of September, San Mateo
Hope it helps (I grew up in Asturias) but have lived in the UK too long now for much better info
Joaquin