Guam has a tropical climate year-round — hot and humid, with temperatures staying between 24°C (76°F) at night and 30°C (86°F) during the day regardless of season. What changes is the rain.
Dry season (January to May) is the best time to visit. Rainfall is lower, humidity is more bearable, and the sky is reliably blue. February to April is the sweet spot — you're comfortably outside the typhoon window, and the weather is about as pleasant as a tropical island gets.
Wet season (July to November) brings more frequent and heavier rain. Afternoons often see brief but intense downpours, and the humidity climbs. Typhoon season overlaps almost exactly: storms can develop any month of the year, but the peak risk runs from August through October. A serious typhoon disrupts everything — flights, tours, and basic services. If you visit during this period, check weather forecasts carefully and know what to do if a warning is issued.
June and December are transitional months — wetter than the dry season but not the worst of the rains. June in particular can be pleasant and is slightly less crowded than peak season.
Crowds follow the Japanese school holiday calendar more than anything else, since Japanese tourists make up the largest share of international visitors. Expect busy periods around New Year's, Golden Week (late April to early May), and Obon (mid-August). Outside those windows, even dry season is manageable. Note that Liberation Day (July 21) is a major local celebration that draws domestic visitors.