Saint Lucia has a tropical climate and stays warm year round, with temperatures ranging between 23-31 °C (74-87 °F). In most parts of the islands, a gentle breeze prevents the air from being both too moist and too hot.
You will enjoy the best weather between December and April, which is the driest season, but also the busiest time. Another option would be to travel between May and June since you will still have excellent weather conditions.
The rainy season goes from June to December, but the weather stays warm and the showers usually don’t last long, which means you can still enjoy your vacation and go for outdoors activities. It is also important to note that it rains more in the rainforest (central part of the island) than on the coasts. This shoulder season is a great time to travel to Saint Lucia too.
Late summer and fall (from mid-August to the end of October) corresponds with the peak of the hurricane season and it would be better to avoid traveling during that time. It is rather unlikely for a hurricane to pass over Saint Lucia, but you might experience a lot of rain and strong winds if there is a tropical storm nearby.