Hi, you're thinking of the Fiestas de San Mateo (note: fiestas with an "s" for plural!) which last for about a week, until St Matthew's Day on 21 September 2018. In the midst of this you also have the "Dia de America en Asturias" (a celebration of the Asturians who departed for and returned from the Americas, big crowds, lots of floats and colourful costumes). The big days will probably be: the weekend following the "pregon" (announcement/ opening of festivities, this year around the 14th), the Day of the Americas on 19/09, a huge fireworks display on 20th, and then all the parades and festivities on Saint Matthew's Day itself.
I was there for a day during the festivities last year (can't remember which day exactly, probably the final day as I remember a procession) - anyway it was a huge amount of fun! SO many people, but also lots of traditional foods and copious amounts of sidra and vino. Lots of folk singing together. Very joyous, and a real cultural experience I'm glad I didn't miss! (p.s. I've had enough sleepless nights due to fiestas in Spain to understand the instinct to avoid, but from what I remember most events are in the city centre around the cathedral (in fact it seems they have a concert in the square until at least 1am for a whole week!), but perhaps you'd be ok if you stayed in a hotel on the outskirts?)