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 Barcelona is January and the most expensive month to fly is February. (Average prices, based on 420 datapoints.)
The best day to arrive at Brussels is Friday and it would be best to avoid Saturday, as prices are on average 86.14% higher than on Friday. (Average prices, based on 533 datapoints.)
The best time to fly is at around 22. (Average prices, based on 659 datapoints.)