Tor said:
Never been there in august, but could the weather realy be worse than in December ? I thougt that was the worst mont in the year. Haven't been much sun in december the last to years in Sosua anyway.
Historically, August is rainier than December, but as you said, Tor, the last 2 Decembers have been horrible. Not only rainy, but outright cold.
The only thing for sure with the weather here is that nothing is for sure.
As the OP wants to party with other single tourists and Cabarete would be the ideal location for him, taking into account that Brit mentioned that Cabarete is quiet in August, then December & March will be his best choices.
December is bound to be a lot more expensive than March.
If the weather in December in the last few years is a trend, then March would be his best bet.