Translation:Maandag zullen zij zijn gekomen.
As a Brit, I find the English sentence weird. I feel it at least needs a word like "by" or "on" at the beginning.
How about, "They will have come Monday"? Or do you still want the 'by' or 'on'?
Yeah, same. It sounds strange to me without the preposition. But again, that could specifically be a British English thing.
I'm American and I would never use it this way without a preposition. I say "on monday" or "by monday" depending on the meaning.
Is this tense often used in Dutch? It seems to me like something that would be used in rather formal situations. But it may be just my impression.
Yeah, it's not used much.
'Zij zullen maandag aangekomen zijn' (arrived) sounds better.