I morgen, bliver det en varm aften.
My guess is that even in Danish it isn't a good sentence and needs a bit of editing.
Well, without the comma I guess this sentence would not be so bad.
Is the second 'aften' in this sentence meant only as a tongue twister or is it necessary?
While grammatically correct, I would question the logical structure of this sentence. No one speaks like that, it should be "It will be a hot night tomorrow".
Why is "warm" incorrect? and why is "evening" incorrect?
Same question here
Also, why is aften translated as night? Night is nat. I would translate this as "warm evening" not "hot night".