Weather in Guatemala varies significantly by altitude and climate zone. For a small country, it has many climate zones, and well as two "seasons," the dry season (November - May) and the rainy season.

There are roughly three climate zones:
1) Areas from sea level to 1,000m are hot, humid, and tropical (such as the Pacific coast and El Peten)
2) areas from 1,000m-2,000m are temperate (much of the country including Guatemala City, Antigua, Coban, Lake Atitlan regions)
3) areas above 2,000m are cool and can dip to freezing at night (Quetzaltenango and northwestern highlands)

It doesn't rain all the time in the rainy season, mostly in the afternoon and into the evening, often for just a few hours. The dry season is also not totally dry, it may rain a few days a month.  In Peten including Flores and Tikal, many might find the duration of May to be uncomfortably hot.  The burning of milpas and the shedding of leaves from decidious trees might not provide the "jungle-like" setting many tourists desire while visiting Peten.

The dry season is the high season for tourists, so you may find higher prices and more crowded areas if you travel then. There is, however, never a bad time to travel to Guatemala.

Unquestionably, the best time to visit Guatemala is from november until late february. The weather can be a bit chilly, but you get mostly clear, blue skies, which allow you to enjoy the beauty of Guatmala's mountains and volcanoes. Probably the worst time is to visit Guatemala between mid march until the beginning of the rainy season (around mid may). The air becomes really hazy, there is very poor visibility...the volcanoes are almost impossible to view, and it's very hot and dry. As soon as the rainy season starts, there are beautiful days and very rainy ones. The rainy season is probably not as good to visit Guatemala as the november-february period, but it's definitely better than the mid march-mid may period. The rainy season stops around the end of october, so normally, by november, you get very little rain, and gorgeous, sunny days.