November to April is the dry season and the most comfortable time to visit. Temperatures sit between 18°C and 26°C, humidity is lower, and the skies are often clear. This is peak season for the whole island, so hotels fill up and prices rise, but Haikou itself rarely feels overwhelmed.
May to October is rainy season. Heat and humidity are high, and Hainan is in the typhoon belt — serious storms can arrive with little warning between July and September. That said, many days during this period are perfectly pleasant, and the island is noticeably less crowded. If you are visiting the temples and the old town rather than the beach, rain matters less.
Spring Festival (late January or February, depending on the lunar calendar) brings the largest crowds of the year as mainland Chinese travel south for the holiday. The Bo'ai Road area is lively but very busy, and transport in and out of the island is heavily booked.
The Hainan International Tourism Island Expo is held annually in Haikou, usually in autumn, drawing trade visitors and some consumer events around the convention centre.