Spring (March-May): Ideal visiting season with mild temperatures (10-18°C), blooming flowers in parks, and fewer crowds than summer. Easter weekend can be busy. The Flower Carpet on Grand Place occurs in August (biannually). Gardens and parks are at their most beautiful.
Summer (June-August): Peak tourist season with warm weather (18-25°C), longest days, outdoor festivals, and terrace dining. Book accommodations early. Expect crowds at major attractions. Many locals vacation in July-August. The Ommegang historical pageant occurs in July on Grand Place.
Autumn (September-November): Excellent visiting time with pleasant temperatures (10-17°C), fall foliage in parks, and cultural season beginning with concerts and exhibitions. September is particularly nice with summer crowds gone but weather still mild. Rain becomes more frequent.
Winter (December-February): Cold and gray (0-7°C) but magical with Christmas markets, festive decorations, and fewer tourists. Winter Wonders transforms the city center from late November to early January. Museums and indoor attractions shine. Bring warm, waterproof clothing. Hotel rates are lowest.
Weather Reality: Brussels is rainy year-round (it rains approximately 200 days annually), so always pack an umbrella and waterproof jacket regardless of season. Weather is changeable - be prepared for sun and rain in the same day.
Best Overall: May, June, and September offer the optimal combination of pleasant weather, manageable crowds, and full accessibility to attractions and outdoor activities.