Late May to mid-June and September to early October are the sweet spots — warm, calm sea, vineyards green or in harvest, and (just) bearable crowds. July and August are hot, packed, and you'll queue everywhere; book trains, restaurants and accommodation well ahead.
December is wonderful and underrated, because of the Andreoli nativity lights on the hill above the village and the much smaller crowds; bring a coat. November and early March are the quietest months — many restaurants close, the sea is choppy, but the village itself is at its most lived-in.
Always check trail status before walking — landslides close paths regularly along this coast.