St Lucia is a great destination but take into consideration the wetter months of June through November and this is also Hurricane Season but you would be unlucky to get hit by one. The more popular events and festivals are during this period. Prices will be lower and the place less crowded too so its a mix really and this applies to the Caribbean in general. Anyway check out this video as a great starter to St Lucia. The St. Lucia Jazz Festival is held in early to mid May.
Castries the capital is a major port of call for cruise ships, which dock at Pointe Seraphine but the town is well worth a visit for its Cathedral and Market.