Mannheim has a relatively mild climate for Germany, sheltered by the Rhine plain and the Odenwald hills to the east. Summers are warm and can be hot — July and August regularly reach 28–32°C. Winters are grey and damp but not especially cold.
Spring (April–May) is a good time to visit: the Luisenpark and palace gardens come into colour, outdoor terrace season begins, and the city is not yet crowded with summer visitors.
Summer (June–August) brings outdoor events along the waterfront, open-air cinema, and an animated atmosphere in the Jungbusch. The heat can be oppressive in August.
Autumn (September–October) has the advantage of harvest season in the nearby Palatinate wine region, making a side trip to the wine route particularly worthwhile. The Luisenpark holds its Illuminationen light festival in autumn — genuinely worth seeing.
Winter has a decent Christmas market on Friedrichsplatz around the Wasserturm, running from late November through late December. Otherwise it is a quieter and less rewarding time.
The Maifeld Derby (usually June) draws an international crowd for a music festival with an eclectic lineup, held on the old military training ground. It has a reputation for a relaxed, well-organised atmosphere.