The tourist high season runs December through March, coinciding with the winter months in most northern countries. It also coincides with Guatemala's dry season. Throughout this season, and especially around the Christmas and Easter holidays, hotels can be booked solid well in advance, so be sure to have a reservation, especially in the more popular tourist spots. Easter and Holy Week are major holidays in Guatemala and in Antigua specifically. Hotels in Antigua are booked solid as much as a year in advance.
In general, the best time of year to visit weather-wise is in December and January, when everything is still green from the rains, but the sky is clear. If you want to avoid the crowds, I recommend traveling during "shoulder" periods, near the end or beginning of the rainy season, when the weather is still pretty good. Note: Some of the country's rugged roads become downright impassable without four-wheel-drive during the rainy season.
Climate -- Guatemala is a tropical country and has distinct wet and dry seasons. However, some regions are rainy all year, and others are very dry and sunny for most of the year. Temperatures vary primarily with elevations, not with seasons: On the coasts it's hot all year, while up in the mountains and highlands, it can be quite cool at night and in the early morning, before the sun heats things up, any time of year. At the highest elevations (3,500-4,000m/11,500-13,120 ft.), frost is common.
Generally, the rainy season (or invierno, winter) is May through October. The dry season (or verano, summer) runs from November to April. Along the Pacific Coast, the dry season lasts several weeks longer than in other places. Even in the rainy season, days often start sunny, with rain falling in the afternoon and evening. On the Atlantic coast, the weather is less predictable, and you can get rain year-round, though this area gets less rain in July and August than the rest of the country. The rainforests of the Petén get the heaviest rainfall, and the rainy season here lasts at least until mid-November. The chart below is for Guatemala City, which has similar temperatures to Antigua, Lake Atitlán, and Chichicastenango. Conditions are different in the Petén, Central Highlands, and both the Atlantic and Pacific coasts.
Holidays -- Because Guatemala is a predominantly Roman Catholic country, most of its holidays are church-related. The biggies are Christmas, New Year's, and Easter, which are all celebrated for several days. Keep in mind that Holy Week (Easter week) is a major holiday time in Guatemala. Government offices and banks are closed on official holidays, transportation services are reduced, and stores and markets might also close.
Official holidays in Guatemala include January 1 (New Year's Day), Thursday and Friday of Holy Week, June 30 (Armed Forces Day), July 1 (Day of Celebration), August 15 (Virgen de la Asunción), September 15 (Independence Day), October 20 (Commemoration of the 1944 Revolution), November 1 (All Saints' Day), December 24 and 25 (Christmas), and December 31 (New Year's Eve).