Dortmund is a year-round city rather than a seasonal destination. The climate is mild and damp — typical northern European lowland weather, rarely very cold or very hot. Summer (June to August) brings the longest days and the best weather for the parks and outdoor spaces, and the Westfalenpark's summer programme includes outdoor cinema and concerts. Spring and autumn are perfectly pleasant, with the Rombergpark botanical garden at its best in April and October.
The one time of year with a distinctive draw is the Christmas market season (late November to Christmas). Dortmund's Weihnachtsmarkt on the Hansaplatz and Altmarkt is one of the largest in Germany and reliably crowded; the city claims its Christmas tree — assembled from hundreds of smaller trees bolted into a conical frame — is the world's tallest decorated Christmas tree. Match-day weekends when Borussia Dortmund are at home (roughly alternate weekends from August to May) transform the city; book accommodation in advance if you want to attend a game.