Medicine Hat is one of the sunniest places in Canada — over 2,500 hours of sun a year, more than most cities in the Mediterranean — and the climate runs hot in summer and cold in winter, with little of the in-between.
June to September is the obvious window: long, warm days, all the parks open, Echo Dale's swimming beaches in use, and the Medicine Hat Exhibition & Stampede in late July if you want a small-town rodeo. July can crack 35°C; bring sun protection.
Winter (November to March) is dry, sharp, and bright, with the occasional Chinook from the Rockies pushing temperatures well above zero for a few days at a time. Walking the riverside parks under fresh snow has its own appeal, but you need proper clothing.
Spring and autumn are short, often dramatic, and uncrowded — good shoulder seasons if you want the prairie light and the migrations through Police Point Park.