The Asturian coast has a genuinely Atlantic climate — green, mild, and wet for much of the year. Summers are warm but rarely hot (average highs around 22°C in July and August), and this is when Llanes fills with Spanish holidaymakers from Madrid and further inland. July and August are the busiest months; beaches get crowded and accommodation should be booked well in advance.
June and September are the best compromise: the weather is good, the beaches less crowded, and the prices lower. Spring (April–May) is beautiful — the countryside is intensely green and the wildflowers are out — but sea temperatures are still cold for swimming. The blowholes at Bufones de Pría are most spectacular in autumn and winter storms, if that is your reason to come. Llanes' own annual festival (La Magdalena) runs in late July; the town is lively and packed.
Rain is possible at any time of year. Bring a waterproof layer whatever the season.