Is 24 - 27 August a good time to go to Sanya? Is it warm and/or does it rain a lot?

I don't think so. Winter from mid-December to late February is the best time to travel there. In August,the average temperature is about 28 centigrade degrees. And it is very hot and humid. You'd better take along short-sleeved shirt, skirt and shorts. By the way, sunglasses, suncreen with high SPF and sun hat are necessary if you go there in August.