Douglas sits at 4,800 feet on the high plains and has the climate that implies: hot summers, cold winters, and wind year-round. Summer (June–August) is the main visitor season. July temperatures reach the low 90s Fahrenheit (around 33°C) during the day, dropping to comfortable evenings. The North Platte is at its best for fishing in late spring and early summer before flows drop.
August is when Douglas is most alive. The Wyoming State Fair arrives in late August and runs for ten days, drawing crowds from across the state. Book accommodation well in advance if you plan to visit during the fair.
Spring and autumn are quieter and perfectly fine for visiting. Temperatures are moderate, the crowds are gone, and Fort Fetterman and the Pioneer Museum are easier to explore at your own pace. Fort Fetterman is only open Memorial Day to Labor Day (Tuesday–Saturday), so plan accordingly.
Winter is cold, with temperatures regularly dropping well below freezing, but Douglas is a functioning town and stays open. The Laramie Range gets snow and offers snowmobile trails in winter. If you are just passing through on I-25, winter is no obstacle.