The best times to visit are in the late spring -- when the northern European days are long and the summer crowds have not yet filled the beaches, the highways, or the museums -- and in fall. The stereotype of overcast Belgian skies is not unfounded, and early spring and fall do get quite a bit of rain, but the weather can change dramatically in the space of a few hours.
Because Belgians take vacations in July and August, summer is a very good time to be in Brussels. In summer you will also be able to get a break on hotel prices; on the other hand, this is also vacation time for many restaurants.
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