Hi, I can tell you Ostend is the greatest cargo airport in Belgium. But when the best time is; can't tell you. Lot of these airplanes are delayed.
But you can see there B707's and DC8-airplanes. There are also sometimes Antonov's, Ilyushin's and many times a week, an Egyptair A300 visits this airport.
In Ostend, there are a few good spottingplaces; some of them near the runway.
I hope you can catch some good birds!