May is the Cannes Film Festival — the city transforms completely, every room is booked, and prices triple. If you want to see the festival, book a year in advance. If you want to avoid the crowds, stay away in May.
June through September is the beach season: warm, crowded, expensive, and reliably sunny. The Croisette is at its most glamorous and its most expensive. September, after the summer crowds thin out but before the weather turns, is arguably the best time to visit.
October through April is quieter and more affordable. The Cannes Yachting Festival in September and MIPCOM (TV and media trade fair) in October bring short bursts of professional visitors. The winter months are mild by northern European standards — daytime temperatures rarely fall below 10°C — and the old town and market are well worth visiting year-round.