Guatemala City sits at 1,500 metres elevation, which gives it a pleasant climate year-round — warm days, cool nights, and nothing like the heat of the Pacific coast. The dry season runs from November to April and is the most comfortable time to visit, with clear skies and no afternoon rain. The rainy season (May to October) brings daily afternoon downpours, usually starting around 3pm and lasting an hour or two, after which the sky clears.
The city's festivals centre on September 15 (Independence Day), which sees parades, marching bands, and celebrations citywide, especially around the Palacio Nacional and the Parque Central. Semana Santa (Holy Week) is celebrated intensely in Guatemala — while most visitors head to Antigua for the famous processions, the capital's own Semana Santa events are large and atmospheric.
There is no strong seasonal reason to avoid the capital at any time of year. It is a working city and functions normally in the rainy season.