Catalina is busiest from Memorial Day through Labor Day (late May to early September), when the weather is warmest and the ferry runs at maximum frequency. The island is genuinely crowded on summer weekends — Avalon's small streets fill up, restaurant waits get long, and the beach at the waterfront is packed. If you're coming in summer, midweek is markedly quieter.
Spring (March–May) and autumn (September–November) are the sweet spots: the weather is mild, the water is clear, and the crowds are manageable. Winter is cold by California standards (daytime highs around 15°C), but the ferry still runs, the island is nearly empty, and some visitors find the quiet more compelling than the busy season. The marine life is excellent year-round.