Welcome summer. (Love your choice of name so I can say what I just did, "Welcome summer." I love summer in Arillas....)
Early September is likely to be hot and summery. But it could just be thunderstorms all week - you take your chances, like you do in UK. But in the main, it is likely to see you sweating (or glowing if you subscribe to those who see it that way!)
Everything will be in full swing in second week of September. There will still be wonderful acts in the Coconut bar, and everywhere will be alive. It is a lovely time to be in Arillas. A bit cooler, a bit more laid back but plenty of opportunities to party, or just chill.
The mosquitos have been busy this year. Spray your feet, ankles and calves, and any bare skin with Jungle Formula (50% DEET) and cover up in the evenings particularly. You are eating gorgeous Arillas food while they are eating you. Jen never gets bitten now with that regime but has suffered big-time in the past.
We often go to Arillas at the very end of Octomber. The sea is always very warm, but might be wild. In early September it is much warmer than early June. This July it was gloriously warm. Yous take your chances but may be lucky. It is never truly cold for all of the season.
Hope your experience is as good as you wish. Jen and I have now spent 11 months of last 12 years there with almost no complaints. Yammas
Phil