Mild weather and several famous festivals, like Tomorrowland, make April through October the best time to visit Belgium. Even though the skies are usually clear, rain can happen anytime.
June through August is peak season in Belgium with invasive chords of tourists and higher rates on accommodation. However, this is also a great time to go horseback fishing in West Flanders, to hike in Ardennes, or to chill on the coast.
Autumn brings in lively festivities to the country, such as the Ghent Festival of Flanders. Going to Ardennes is a good idea if you are an adherent of hunting.
In the winter, try cross-country skiing. Bruges and Ghent are considered to be the best ski resorts in Belgium, so there is an excellent reason to adore the cold season.