Winter is the most popular time to visit San Juan as the frozen masses from colder climes come to thaw out. Old San Juan's streets fill with cruise-ship passengers and bookings are essential at the hottest hotels and restaurants. The boisterous street party of Festival San Sebastián in mid-January is a high point, especially for locals.
At other times of the year, San Juan is much quieter, even though the beaches remain alluring year-round. From April to October you'll find plenty of deals and diminished crowds.
From December through to May, the weather is at its best – highs in the mid-80s, lows in the high 70s and little humidity.