but that would not be right - it is the last full week in september from Monday through Saturday, so dates change annualy.

So you want to change the entry every year. Why not just leave it like it is? (as it is for the last 5 years)

the fun park is only existent (=open) when the "Windhoek Show" (largest Namibian trade fair) takes place... and that is during last week of September.