the day before, the previous day
我前一天丢了一台相机。
I lost a camera the day before.
他们前一天就作了准备。
They made preparations the day before.
我还发现，西尔弗·穆恩前一天下午就已经回来了，比预计早了三天。
I also found out that Silver Moon had already returned the previous afternoon, three days earlier than expected.