Special terms for days:

One day: 하루

Two days: 이틀

Three days: 사흘

Four days: 나흘

Five days: 닷새

Six days: 엿새

Seven days: 이레

Eight days: 여드레

Nine days: 아흐레

Ten days: 열흘

Eleven days: 열하루

Twelve days: 열이틀

etc.

하루 and 이틀 are pretty common. 사흘 and 나흘 less so. Nobody ever uses 댓새 and beyond. I've been told this by many Koreans. They always tell me after I use a word like 아흐레 or 엿새 in conversation. I actually wind up confusing them at first because those words are so rare.