七日 is the literary or formal way to say 'seven days' / 'a week'; in casual speech use 一个星期 or 七天. Often appears in legal notices (七日内 'within seven days') and Buddhist contexts (the 7-day mourning intervals 头七 to 七七).
culture
In Chinese folk tradition, the seventh day after a death is 头七 (tóuqī), when the deceased's spirit is believed to return home for a final visit. This makes 七日 culturally weightier than just a calendar week.
