Luxembourg City has a mild oceanic climate, more like Belgium than the Mediterranean. Summers are warm but rarely hot (averages 18–24°C), winters are cool and damp with occasional snow. The driest months are usually April and May.
May to September is the best window for walking the casemates, the Pétrusse park, and the Chemin de la Corniche. June and September are particularly pleasant — busy but not packed. July and August are warmer but bring more thunderstorms.
The city has two events worth timing a visit around: the Schueberfouer (a large funfair that has run since 1340) takes over the Glacis from late August to mid-September; the Christmas markets across Place Guillaume, Place d'Armes, and Place de la Constitution run from late November to Christmas Eve and are the prettiest of the year. October and the long winter half are quiet and many sights have reduced hours.