The temperature in the U.S. Forest Service’s only tropical rain forest—an easy and popular day trip from San Juan—is about 73°F year-round, so seasonality isn’t an issue. It can't be said enough, though: come prepared for rain. Showers are a factor every day. Indeed, the park receives an estimated 200–240 inches of rain annually.
Prior to Hurricane María, when the forest was fully operational, it had more than a million visitors each year. Ongoing post-storm restoration and a gradual return to normal operations have led to a drop in visitations, but the forest can still get crowded, with fewer areas open to accommodate those who do come. If you’re driving yourself, try to book a morning timed-entry slot (on recreation.gov), and arrive early to readily find parking. If you're taking a guided tour, book a morning departure to avoid larger crowds.