Kiel has a maritime climate — mild, damp, and often windy. Summers are warm but not hot; winters are grey and cold but rarely severe. The sea moderates temperatures year-round.
Late June is the peak time to visit if you want to experience Kieler Woche, the world's largest sailing regatta. The ten days of racing and festivities at the end of June transform the city. Accommodation prices double and rooms fill weeks in advance — book early. The sailing itself is spectacular even if you know nothing about the sport; the sheer density of yachts on the fjord is worth seeing.
Summer (July–August) is pleasant for the fjord and the beach at Laboe. The Kiellinie and harbour areas are busy with day-trippers from Hamburg. Daylight extends well into the evening this far north.
Spring and autumn offer quieter visits and reasonable weather. The ferry crossings to Scandinavia are atmospheric in any season, but rougher in autumn and winter when the Baltic can be genuinely unpleasant.
Winter is quiet. Some attractions reduce their hours or close, and the harbour is subdued. But the Christmas market on the Rathausplatz is well regarded, and the city functions perfectly well through the cold months.