The 8th day of a month (date). Read as ようか (yōka).
今日は８日だから、支払い日だよ。
Today is the 8th, so it's payment day.
ミーティングは来月の８日に予定されています。
The meeting is scheduled for the 8th of next month.
彼の誕生日は八日（ようか）だ。
His birthday is on the 8th.
Can indicate a period of eight days (i.e., a duration of eight days), depending on context.
工事は全部で８日かかった。
The construction took eight days in total.
旅行は８日間の予定だ。
The trip is planned for eight days.
入院は８日で済んだ。
My hospital stay was over in eight days.