I don't know about the rest of Sardinia, but June is most definately peak time at the Cala. We were last there at the end of May last year and the entire staff was in effect "setting up and gearing up" for the mad rush of summer.

I believe peak time at the Cala is from about mid-June to mid-August. Late May was just slightly too early as it was just a touch too cool to be comfortably around the pool or on the beach without being directly in the sun.

I understand early/mid September is one of the best times to go, still warm and much less crowded than during the summer.