Why does it not translate to "That opens on Monday" as well as "It opens on Monday"?
In this case they're referring to a common ("en") object. For example: Jeg har en bog. Den åbner på mandag. (okay that doesn't make much sense but you get the point)
The discussion suggests that the correct translation is "It opens on Monday. " That's what I put but it was marked wrong and says that it should be "It's opening on Monday." I've checked two translation sites and both translated "Det åbner på mandag" as "It opens on Monday." When I put in "It is opening on Monday," the answer was "Det åbner på mandag." It would appear that both are correct.