July and August are the peak months. The beaches fill up, accommodation needs to be booked weeks or months in advance, and the roads on the peninsula — especially the narrow lanes to the National Trust car parks — can be slow. That said, the weather is at its best and boat trips to Bardsey Island run reliably.
May, June, and September are the best times to visit. The weather is decent, the crowds are manageable, and the countryside is at its most vivid. June is particularly good — long days, wildflowers on the headlands, and the gorse still in bloom. September brings quieter beaches and often surprisingly warm water after a summer of sun.
Winter is not unpleasant but many seasonal businesses close, boat trips to Bardsey stop entirely, and the peninsula's intrinsic character — windswept headlands, empty beaches, a strong sense of the edge of the world — is at its most pronounced. If you want Porth Oer to yourself, come in November.
The Llŷn Peninsula is one of the sunniest parts of Wales — it protrudes into the Irish Sea and catches weather from multiple directions, but it also gets more sunshine hours than much of North Wales. Rain is always possible.