The sense of the word/phrase 'goedendag' I'd like to explore to day is when it is used as a noun. In this sense it is a 'mace' (a medieval weapon). See more of the wikipedia article here. When I was reading this article one of the explanations of this word is "good dagger" (the other one seems a bit spurious to me!) so this sent me off on a research hunt!
dagger = dolk
In modern Dutch this would be 'goede dolk', so where is the link?
dolk sounds more like the Scottish word 'dirk' for dagger or short sword and sure enough the etymology shows common roots for dirk and dolk: " the earliest spellings (of dirk) were dork,durk [possibly from] Ger. dolch "dagger.""
Ok so that's sorted out where dolk came from but why would a mace be called a goedendag then (if the theory of it referring to a dagger is true)?
More research shows that the English word 'dagger' comes from middle Dutch 'dagge' and would line up nicely with a mediaeval weapon that looks like the one pictured here: a good dagger indeed.