Brussels works year-round but has two clear high points. May to September is the best weather, with long warm days and a full programme of outdoor events. The city is busiest in July and August, when it hosts the Ommegang (a historical pageant on the Grand-Place in July) and the Flower Carpet (biennial, August of even years — half a million begonias on the Grand-Place). Summer is also when the terrace culture takes over every square in the city.
December is worth considering for the Christmas market, which spreads across the Grand-Place and Place Sainte-Catherine with atmospheric lighting and a better-than-average range of food and drink stalls. It is cold and often rainy, but hotels are cheaper than in summer.
Spring (March–May) is the time to see the Royal Greenhouses of Laeken — the vast iron-and-glass structures on the royal estate are open to the public for only a few weeks each spring when the exotic plants are in flower. The timing changes annually; check the Royal Palace website ahead of your visit.
Avoid the European Parliament session weeks in autumn if you want reasonable hotel prices — MEPs and lobbyists fill the EU quarter and push rates up sharply across the city.