Hamburg has a maritime climate — wet, windy, and rarely extreme. Westerly winds bring rain off the North Sea year-round, so a good raincoat matters more than the season you choose.
Late spring and early summer (May to early July) are the best months: the chestnuts are out along the Alster, the days are long, and the rain is still relatively light. The driest weather of the year is February to May, but the city is bleak in winter. High summer (July and August) sees temperatures around 22°C with longer warm evenings, but also the year's heaviest rainfall — the gardens are at their best, and the harbour is busy with sailing on the Aussenalster.
Autumn is grey and gets dark early. Winter is properly cold; January averages around -2°C and the lakes sometimes freeze over enough for skating. The compensation is the Christmas markets on Rathausmarkt and at Jungfernstieg, full of Glühwein and gingerbread.
Festivals worth timing a trip around include the Hafengeburtstag (Harbour Birthday) in early May, when tall ships and 1.5 million visitors fill the harbour for three days; the Hamburger DOM funfair (three times a year, spring/summer/autumn) on the Heiligengeistfeld; the Reeperbahn Festival in late September, Germany's biggest live-music showcase; and Cruise Days in early September, when the cruise terminals light up after dark.