The Florida Keys have two distinct seasons that matter for planning a visit to Big Pine Key.
Winter (December through April) is the dry season and the most popular time to visit. Temperatures are mild — daytime highs in the low-to-mid 70s Fahrenheit — humidity is low, and the trade winds keep things comfortable. Water visibility on the reef peaks in winter and spring, making it the best time for diving and snorkelling at Looe Key. Accommodation prices are at their highest and demand is greatest: Bahia Honda campground reservations six months in advance are not unusual.
Summer (June through October) is hot, humid, and prone to afternoon thunderstorms. Water temperatures rise into the upper 80s Fahrenheit. Hurricane season runs June through November, with the peak risk between August and October. That said, summer has real advantages: accommodation prices drop significantly, the crowds thin out, and the tarpon fishing in May through July is extraordinary. The reef is still very diveable between storm systems.
Spring (April and May) offers a balance — reasonable prices, good weather, and the beginning of tarpon season. Fall (October and November) can be very pleasant once hurricane season settles, and prices remain off-peak.
The Winter Star Party, an annual gathering of amateur astronomers, typically takes place at Scout Key in February. Big Pine Key's dark skies — far from any major city — make the lower Keys a genuine destination for stargazers.