Kokura is liveable year round but most enjoyable in spring and autumn. Late March to mid-April brings cherry blossom around the castle moat — one of the best urban viewing spots in northern Kyushu. October and November are dry, cool, and ideal for walking the arcades and visiting the gardens.
Summer (July to early September) is hot and very humid, with a real risk of typhoons in late August and September. Winter is mild by Japanese standards — temperatures rarely below 5 °C — and the city's huge underground network of arcades and shopping concourses makes a rainy day no obstacle.
The Kokura Gion Daiko festival in mid-July fills the streets with hundreds of drummers; if you can time a visit around it, do.