I assume that it should be “solnedgången” with an “en” at the end…?
No, only in Gothenburg
Joking aside, I can’t tell for sure why this seems to be an exception to the rule, It might be because of the (here omitted) “som”. Compare “den vackraste solnedgången är när solen är röd”.
Hej morbrorper,
thank you for your reply. This is an area of Swedish grammar which is really difficult to me. I remember now that someone from Sweden once told me that two markers of definiteness suffice, i.e. in “Det här är den vackraste solnedgång jag någonsin sett” the word “den” is the first marker, and the relative clause is the second, so we do not need a third. But I am not sure whether that is the key here. But it would explain the difference to your sentence “den vackraste solnedgången är när solen är röd” because there is no relative clause as a (second) marker here.