Translation:It takes seven days.
Etymology doesn't have much to do with why languages develop exceptions. That usually has to do with the physiology of pronunciation: the grammatical way to pronounce a thing might be mildly physiologically staining, so over the years, native speaker will unconsciously take shortcuts that over time become the norm.
～日【か】counter for days (date)
more about days and months pronunciations here:
This will definitely get lost, but if anyone has trouble with memorizing different readings of numbers in dates, I recommend WaniKani. It teaches you kanji and vocabulary. It's free up to 3rd level and on the 2nd you learn dates. I'm not promoting, just genuinely trying to help fellow learners