Translation:The Danish youth only eats koldskål in the summer.
Here's a picture to help you http://sofiesspisekammer.dk/wp-content/uploads/Koldsk%C3%A5l-f%C3%A6rdig2-juli-20131.jpg
Om sommeren refers to just as specific a summer as "in the summer" does, which... generally isn't very specific. However, there's another, more literal translation for "during summer", which is under sommeren (yes, with the definite form). Maybe that's why it didn't take your suggestion.
Names of meals are alway pretty hard to translate, since each region comes up with their very own versions for which other regions have no names. So typically the name just gets adopted, like "pizza", "baguette", or "quesedilla". Koldskål is usually just called "koldskål" or "koldskal" in English.