We do not have enough data on this route to provide a reliable graph showing how many days before departure it is the best to book.
However, usually the closer it is to the departure, the more expensive the tickets are.
The most popular airlines on the route Brussels - Dortmund: Lufthansa (LH).
The cheapest flights you can book on the route Brussels Intl Airport (BRU) - Dortmund Airport (DTM).
There is no direct flight on the route from Brussels to Dortmund
The shortest flights with change last about 4h 55min.