June to September is beach season, and July and August bring Italian summer crowds — the beaches fill, the harbour is busy with boat trips, and the village restaurants run at capacity. The sea is warm from late June through early October.
May and June before the main crowds arrive are arguably the best weeks: the sea is warming up, the national park is green, and the village is not overrun. Wildflowers on Monte Circeo are good in April and May.
September and October are excellent for hiking — the summer heat has broken, the trails are quiet, and the sea is still swimmable. October is ideal for birdwatching, with migrating raptors using the Monte Circeo promontory as a stopover on their crossing of the Tyrrhenian.
Winter is very quiet. Most restaurants and beach facilities close from November through Easter. The old hilltop village has a certain gloomy charm in winter, but there is not much open.