three-day-off vacation/holiday.加了连词符号的是形容词,不变复数.-off可要可不要.这个短语have three-day off 中间要加a吗固定短语是 have some days off,休息几天，歇几天，不加 a，如 have two days off,休息两天。但你这的vacation是名词，要用形容词来修饰，所以要加a。不懂可追问，谢谢采纳。