七日 is a formal or written equivalent of 七天 (qī tiān), meaning 'seven days' or 'a week' (as a time span). It is also used as a date expression meaning 'the seventh day' of a month, particularly in calendars and historical references. In everyday speech, 七天 is more common; 七日 is found in schedules, medical dosing, itineraries, and literary or official contexts.