The climate in Puerto Rico [1] is classified as tropical marine, which means it is sunny, hot, and humid year-round. The only seasonal change in Puerto Rico is in the amount of precipitation that falls.
Rainy season is May–October, which is also hurricane season. A daily shower can be expected, although they don’t typically last very long. Airline ticket prices and hotel room rates are usually discounted during this time.
Dry season is January–May, which coincides with the tourist industry’s high season in most parts of the island, including San Juan [2]. Note that rain does occur during dry season, but at a much lower rate than in summer and fall.
Some parts of the island have a high season that differs from the norm. Because surfing is its main attraction, Rincón’s high season is December–April, when the swells are highest. In Boquerón and other places that cater to Puerto Rican tourists, the high season is during the summer when children are out of school.
Links:
[1] http://www.moon.com/destinations/puerto-rico
[2] http://www.moon.com/destinations/puerto-rico/san-juan