The Channel Islands have one of the mildest climates in the British Isles. Winters are cool rather than cold — temperatures rarely drop below 5°C — and summers are warm and sunny, with July and August averaging around 20°C. The islands receive more sunshine hours than most of the UK: Jersey in particular averages around 1900 hours of sunshine a year, comparable to the south of France.
The best time to visit is May through September. June and September offer good weather with fewer crowds than the peak of July and August. Spring is pleasant for walking — the countryside is green and wildflowers are out along the coastal paths. Autumn brings the Tennerfest food festival on Guernsey and Jersey, when restaurants offer fixed-price menus at reduced rates.
July and August are peak season: accommodation fills up, ferry crossings should be booked well in advance, and the beaches can be crowded. If you're visiting specifically for the beaches, this is still the time to go — the sea water reaches around 17–18°C and is comfortable for swimming.
Winter is quiet. Many smaller restaurants and some attractions close or reduce hours. Ferry services thin out and can be disrupted by weather. That said, the islands are calm and walkable year-round, and accommodation is significantly cheaper off-season.