I visited Brussels in May and fell in love with the city.
I am planning a return visit for Halloween and was wondering do the Belgians celebrate Halloween and if so in what way (Fancy Dress, Fireworks, etc)?
It is getting to be celebrated more and more. This is a direct effect of the appreciation of Hollywood movies and the creative usage of that appreciation by shops. So yes you would see more and more advertisement trying to convince people to celebrate Halloween, since it means top dollar in terms of turnover.
You might be invited to Halloween parties and then as a native halloween celabrator your way of how to celebrate Halloween would be appreciated as model for future celebrations.
In terms of the general public, it is not a big thing.