Summer (June through August) is the easiest visit — long days, warm but rarely humid afternoons, and the river and Casper Mountain at their best. Wildflowers peak in June. July can be very hot in the city; head up the mountain to escape.
September brings cooler evenings, golden aspen, and the best stretch of trout fishing on the North Platte. Winters are long and seriously cold — January highs hover around -3°C and the wind makes the rest feel worse — but the cross-country skiing on Casper Mountain is excellent. The city briefly drew worldwide attention on 21 August 2017 when it sat directly in the path of a total solar eclipse; the next chance won't come around for centuries.