Fajardo in October

December through May gives you the sweet spot — dry weather, calmer seas, and the bio bay glows brightest in cooler water. January and February are perfect, with highs in the low 80s and almost no rain. But here's the thing about timing your bio bay tour: you want a new moon. The darker the sky, the more dramatic the bioluminescence. Plan your trip around the lunar calendar if this is your main reason for coming. Summer gets hot and humid, plus afternoon thunderstorms that can cancel boat tours. Hurricane season runs June through November, though direct hits are rare. September and October see the most storm activity. Weekends year-round bring crowds of San Juan locals to the beaches. If you want Seven Seas to yourself, come on a Tuesday morning. Christmas and Easter weeks get absolutely packed. Hotel prices double and good luck getting a bio bay tour without booking weeks ahead.