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 cheapest flights are on the route Barajas (MAD) - Dortmund Airport (DTM).
There is no direct flight on the route Madridból Dortmundba
Il volo con scalo più breve dura 10h 10min.