Chipiona is a summer town in the most traditional sense: July and August are when Spanish families take their month at the beach, the town's population triples, and every restaurant runs at full capacity. May, June, and September are quieter and arguably better — the water is warm enough, the beach less crowded, the chiringuitos still open.
Two events worth timing a visit around: the Carnival in February (smaller than Cádiz's, but with the same satirical chirigotas) and the Romería de Regla on the second Sunday of September, when pilgrims on horseback fill the streets and the town stays up most of the night.