Belgium rewards visits year-round, though the weather is temperate and unremarkable — expect grey skies and rain in any season. The country makes up for it with indoor pleasures.
Spring (April–May) is the best time for visiting the Flemish art cities. Crowds are manageable, the weather is mild (12–18°C), and the countryside is green. Brussels Jazz Weekend (late May) is worth planning around.
Summer (June–August) is lively, with long days, outdoor terraces, and the festival season in full swing. The Ghent Festivities (Gentse Feesten) in late July are one of the largest free street festivals in Europe — ten days of music, theatre, and revelry across the entire city centre. Tomorrowland, the massive electronic music festival, takes place in Boom near Antwerp each July. Bruges gets very crowded in summer; if you're going, stay overnight rather than arriving on a day trip from Brussels.
Autumn (September–October) offers the chance to explore with fewer tourists, comfortable temperatures, and beautiful foliage in the Ardennes. The Brussels Beer Weekend in early September is a highlight.
Winter (November–March) is cold and damp, but Belgium's Christmas markets are genuinely good — Brussels' market on the Grand-Place and along the Boulevard Anspach runs from late November through January and draws large crowds. The Carnival of Binche in February or March (depending on the liturgical calendar) is a UNESCO-listed event worth seeing.
Belgium's museums and restaurants are open year-round, so even a January visit to Bruges or Ghent is worthwhile.