The beach resorts are the most crowded and expensive December through Easter, especially the Christmas/New Year's holiday and the weeks before and after Easter. Despite the humidity, upper-class Mexican families book resort hotels during July and August school vacations while the masses rent bungalows in the smaller beach towns and camp out on popular beaches. Mexicans also travel over extended national holiday weekends, called puentes (bridges).
Holy days and cultural festivals play a big role in Mexican life. If you plan to travel during a major national event, reserve lodgings and transportation well in advance. November brings the PV Film Festival of the Americas and International Gourmet Festival.
On the same latitude as the Hawaiian Islands, Puerto Vallarta is tropical, and can be visited any time of year. Mid-June through mid-October is the rainy season; afternoon showers clear the air and temporarily reduce humidity. Rainy season temperatures are often in the 80s and 90s Fahrenheit (20s and 30s Celsius) and feel hotter due to high humidity. November through May is the dry season. Summer means bathtub-like ocean temperatures, the best diving and snorkeling conditions, and the most surfable waves. December through March brings the coolest temperatures: daytime temps still reach the 80s but at night drop to the 50s or 60s. May, June, August, and September are the hottest months. The proximity of tall mountains to the coast increases humidity: from Puerto Vallarta north to San Blas there's jungly terrain (officially, tropical deciduous forest). South of PV, the mountains recede from the coast, making that area's thorn-forest ecosystem drier.
U.S. National Weather Service (weather.noaa.gov/weather/current/MMPR.html). The Weather Channel (www.weather.com).