The best time to visit Cologne is April to May or September to October, when temperatures are pleasant (14–22 °C), crowds are manageable, and outdoor café terraces are in full swing. Spring also brings the Rhine in bloom, with flowering trees lining the promenade.
Summer (June–August) is warm and lively — peak season for river cruises and outdoor festivals — but hotel prices rise and the cathedral crowds thicken. The city rarely suffers the sweltering heat of southern Europe, making it a comfortable warm-weather option.
Winter has two unmissable draws: Karneval, which peaks in February/March and turns the entire city into a costume party, and the Christmas Markets (late November to late December), which are among Germany's finest and most atmospheric. Expect cold temperatures (0–6 °C) but excellent festive spirit. January is the quietest and cheapest month — fine for museum-hopping if you don't mind grey skies.