Luxembourg has a temperate, maritime climate — mild but overcast for much of the year, with rain distributed fairly evenly across the seasons. It rarely gets very hot or very cold.
Spring (April–May) is excellent for hiking and cycling. The forests are coming into leaf, wildflowers are out in the Mullerthal, and crowds are thin. Weather is unpredictable but rewards patience.
Summer (June–August) is the most popular time to visit. Luxembourg City is lively, outdoor terraces are packed, and the hiking is at its best. July brings the Festival de Wiltz and Fête de la Musique (June 21st). Expect more tourists, particularly in peak July and August. Temperatures are pleasant rather than oppressive — rarely above 28°C.
Autumn (September–October) is arguably the best time of year. The Moselle wine harvest is underway, the forests are turning, and tourist numbers are lower. The Grevenmacher Wine and Grape Festival happens in September. Hiking in the Mullerthal through autumn foliage is outstanding.
Winter (November–March) is quiet. Luxembourg City's Christmas markets in December are among the best in the region — Place d'Armes, Place de Paris, and Place de la Constitution all host markets, with ice skating and light installations. January and February are grey and cold, with occasional snow in the Ardennes.
For most travellers, May, June, September, and early October offer the best combination of weather, manageable crowds, and open attractions.