The best time to visit the Cayman Islands is wintertime, which runs from December to April. In this period, the temperature and humidity are in balance: around 24°C (75°F) and the humidity at its lowest point. In terms of events, May is the best time of year to visit Grand Cayman, mainly because the Batabano Carnival, which is a massive street fair. The other main annual event is Pirates Week, which is in the first half of November.
The summer season is from May to November and is also the rainy season. Temperatures rise to around 28°C (83°F) but due to the humidity it can become uncomfortably hot, especially in July and August. Expect to get everyday afternoon showers, although they disappear as quickly as they arrive. October is the worst month with regards to rain. Also beware of hurricane season, which starts in June and runs through the end of November. Note that during Spring and Autumn the Plankton blooms, reducing visibility.
Due to their tropical location, the Cayman Islands have the highest exposure to hurricanes than any other area in the Atlantic. On average, it is affected every 2.23 years, being either directly hit or indirectly affected. On a positive note, crowds are gone and pricing is almost half of that in high season.