sennight
noun·/ˈsɛn.naɪt/
A period of seven days, reckoned as seven nights. A week named by darkness rather than daylight. Sennight feels calendrical and old, time measured in nights huddled together.
He would return in a sennight, he said, and the promise sounded like something written in oak and ink.
Etymology
From Old English seofon niht "seven nights," later contracted. Like fortnight, it preserves a habit of counting by nights, an older arithmetic, winter-minded and practical.
Related Words
fortnighttwo-week span; the most familiar kin
weekthe modern standard term
ychthemerona learned “day-and-night” unit; far more technical
bygoneoften the atmosphere that sennight evokes