Please be aware that Brussels has multiple airports and that these results are for all airports serving the city.
The best month to fly between Brussels and Lisbon is January and the most expensive month to fly is February. (Average prices, based on 404 datapoints.)
The best day to arrive at Brussels is Wednesday and it would be best to avoid Friday, as prices are on average 32.72% higher than on Wednesday. (Average prices, based on 449 datapoints.)