Hannover has a mild but unpredictable maritime climate — summers in the high teens to mid-twenties Celsius, winters around freezing with frequent grey skies. Late spring through early autumn (May to September) is the best stretch, when the Herrenhausen Gardens are in flower, the Eilenriede is green and the Maschsee opens its swimming areas.
Time your visit around an event if you can. The Maschseefest (late July to mid-August) turns the lake shore into three weeks of food stalls, music and fireworks. Schutzenfest, billed as the world's largest marksmen's festival, takes over a chunk of the city in late June or early July. The Hannover Messe (April) and CeBIT (when running) draw enormous crowds and push hotel prices up dramatically — avoid these dates unless you are coming for the fairs themselves.