Firstly if its like last year it is only one of a couple of Dutch Open meetings.Last years riders at Dohren includedJannick de Jong,Jörg Tebbe(D),Henk Bos,Sjörd Rozenberg,Henry van der Steen,Shane Colvin!,(might be able to get more info from him);Lubos Tomicek and others.In all there was over 70 riders and 61 races,so i think it must have started relatively early!
I have a telephone no. +31/263528515 but cant find anything about the meeting in Bahnsport maybe because the Ice speedway final is on in Berlin that weekend.Also i think that the Dutch Speedway Championship is on 18th Sept in Blijham:good luck!