You have multiple options to reach Guatemala by land, sea, or air. Presumably, unless you're arriving by cruise ship or entering from a neighboring Central American country, you'll be arriving by air. Then it will be time to complete entry requirements; generally, this is little more than a valid passport and proof of onward or return travel.
Predictably, the weather you will encounter during your time in Guatemala depends not only on the time of year you visit, but on the microclimate of the specific geographic region you find yourself in. The dry season lasts from November to early May. Although it can sometimes rain during this season, it’s far from normal (except on the Caribbean Coast, which can see rain throughout the year). The coldest months are December through February—the daytime highs in mountainous areas around Guatemala City, Antigua, and Quetzaltenango are usually in the mid-60s. After February, the country begins to warm up as it progresses toward the rainy season. The dry season is the best time of year for hiking and extended outdoor adventures. The temperatures are pleasant and you’re virtually ensured to remain rain-free.
The rainy season typically starts near the end of May and lasts until early November. Days during the rainy season often see showers in the afternoon and early evening, with cloud-free skies during the morning and late evening. September and October are the wettest months. The temperatures are warmer during this time of year.
The rainy season can extend into December in Petén. If you want to visit Petén’s more remote ruins, it’s best to go between February and May, otherwise you could be walking through thick mud.
The rainy season can be a great time to travel in Guatemala—there are fewer foreign visitors and many hotels offer discounts. The country is also much greener during this time of year. If you can cope with a few hours of rain each day, traveling in the rainy season can definitely be enjoyable. Regardless of what time of year you come, make sure you pack accordingly for both weather and activities.
The high tourist season lasts from December to Holy Week (typically in April). During this time of year, there are more travelers in Guatemala and hotel rates go up accordingly. Granted, this tends to be true of any major holiday or festival within the country. Guatemalan schools go on vacation from mid-October until January, and many Guatemalan families tend to take trips during this time of year. Popular Guatemalan destinations can fill up with locals.
There is a second high season from June until early September, which corresponds to American and European summer vacations. The language schools in Antigua and Quetzaltenango are often full of college students during this time of year.