El Salvador's peak seasons are Semana Santa, or Holy Week, which precedes Easter Sunday; the month of August; and mid-December through Christmas. Prices during these times can be higher, but not always. Some hotels actually run specials to keep up with the competition; it just depends on how busy the hotel thinks it will be. Either way, you need to book any decent hotel well in advance during these times, or you won't get a room.
December is actually the best time to visit El Salvador, when the rainy season has just ended and the landscape is still green and the air clean and fresh. The rainy season (Mar-Nov) is the best time to surf, though the dry season is still good and more suitable for beginners. Wildlife-watching is best in the dry season, especially since some parks, including Montecristo, are closed during the rainy season. Turtle-watching is the exception: Nesting takes place from May to November.
Climate -- The country has two distinct seasons in terms of weather. The first is the dry season, which runs from November to April. The second is the rainy season, which runs from May to October. Since there is little temperature variation between these seasons, the question of which season is best for travel is not a simple one. The short answer would be November, when the rains have stopped but the landscape has not yet dried out. However, both seasons have something to recommend them. In the dry season, the country's predominately dirt secondary roads are easier to navigate -- some roads are impassible without a four-wheel-drive during the rainy season -- and, well, it's not raining. In the rainy season, on the other hand, El Salvador's environment is at its most lush and alive. Rainy season also doesn't necessarily mean all-day downpours: The country's highest elevations do receive daily rain and are often covered in a misty fog, but rainy season in the lower elevations can mean little more than daily afternoon showers.
Temperatures throughout El Salvador vary more according to elevation than season. The beaches and San Salvador can get up into the high 80s°F (low 30s°C) year-round, with even higher heat waves in the summer, while the coldest mountains can fall to near freezing, with averages of 54° to 73°F (12°-23°C) year-round. The coldest month is December, and the hottest month is May.
Public Holidays -- Public holidays in El Salvador include New Year's Day (Jan 1); Semana Santa (Holy Thurs-Easter Sun); Labor Day (May 1); the Festival of El Salvador (Aug 1-6, though the rest of Aug remains a busy vacation season); Independence Day (Sept 15); Día de la Raza (Oct 12); All Souls' Day (Nov 2); and Christmas celebrations (Dec 24, 25, and 31).
For an exhaustive list of events beyond those listed here, check http://events.frommers.com, where you'll find a searchable, up-to-the-minute roster of what's happening in cities all over the world.