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.

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.