When is summer in Guatemala?

Actually, just as far as i know, Guatemala has a Spring-Like, mild to alpine cold (In the highlands), and mediterranean to humid (in the low elevation lands, and coast) year long, and even though no seasons are really MARKED, there are some few climatic events.

"Summer" as they call it,--usually brings a rise in Minimum temps, with Maxs almost always staying the same, you can expect some hot weather-- Begins around the last 5 days of February, and lasts all the way to mid of May. Usually in the Highlands, Like Guatemala City, during Summer, Minimums don't go below 55 F. Xela isn't that affected, with Minimums remaining as low as 30 F, usual 34 F, but days do get hot and dry. Maxs may go all the way to 90 F, but as mentioned, they usually stay the same, just that they last longer. The remaining of the country's lands, lowlands present some Stable 66 F to 95 F.

Then some really rainy months. From Mid May, to Beginning of October, the heavy rain season starts --Hence the "Como Agua de Mayo" say....--. Guatemala has problems to cope with this rain, and almost every year there's damage, floods, deaths, etc. There's not a single day without rain, and sun's almost gone for those 5 months. Temps start to get low, with day minimums in Guatemala City no surpassing 70 F, and in Xela not surpasing 65 F. And then we've got It's really windy in Guatemala City.

Down in the lowlands the temperature go really below usual, such as a 64 F in Zacapa.

Starting October, rains diminish, and then the cold begins. Expect lows as 49 F for Guatemala City, 25 F in Xela. Days are humid after all that rain, but begin to dry as entering January. Days maximum don't go above 65 F in both cities. Guatemala City's windy, so that's a wind chill factor.

I've used Guatemala's 2 most important Cities, Guatemala City and Quetzaltenango (Xela). At 1500 Meters and 2300 Meters respectively. So that may not represent the whole Country's climate. Actually, it's always quite cold in those cities. You'd be surprised, so don't expect to go in shorts and soak up margaritas as you'd do in Costa Rica. Take winter clothes, ALWAYS.

The rest of the country (Below 900 Meters) is quite humid with stable temperatures.
Volcan Tajumulco 4200 Meters, and Town of Ixchiguan 3700 Meters, usually get some snow around November-January. Xela gets hail, sometimes snow if precipitaion occurs, which is really weird for october-february, when it's coldest.

And classes start in January, with a 1-2 week break in SEMANA SANTA, then another break in June-July, usually 3 weeked, and then all the way till mid october.