Please be aware that Brussels and Frankfurt has multiple airports and that these results are for all airports serving the city.
The best month to fly between Brussels and Frankfurt is August and the most expensive month to fly is April. (Average prices, based on 256 datapoints.)
The best day to arrive at Brussels is Friday and it would be best to avoid Tuesday, as prices are on average 39.18% higher than on Friday. (Average prices, based on 256 datapoints.)
The best time to fly is at around 13. (Average prices, based on 487 datapoints.)