May, June, and September are the best months to visit Argostoli and Cephalonia. The weather is warm and settled, the island is not yet overwhelmed with visitors, prices are lower, and the landscape is at its most varied — wildflowers in spring, golden hills in early autumn. October is increasingly popular with travellers seeking a quieter experience and remains warm enough for swimming well into the month.
July and August are peak season: very hot, crowded, and expensive. The main beaches can feel overwhelmed. If you're coming in high summer, book ahead and arrive in Argostoli rather than a beach resort — the town has its own rhythm that functions independently of the tourist rush.
Cephalonia's Carnival season (Apokries) in February and March features some of the most elaborate masked processions in the Ionian Islands — more Italian in character than the Athenian carnival. The 12 August commemoration of the 1953 earthquake is a solemn civic event rather than a tourist occasion, but it explains why the entire town pauses on that date.