on certain day什么意思？

在确定的那一天

certain有某些的意思，所以这个短语应该是在某一天的意思。
我的金山词霸上也说明了一点

certain
[ˈsəːt(ə)n, -tɪn]
adjective
able to be firmly relied on to happen or be the case
有把握的，可靠的，无疑的，肯定的
it's certain that more changes are in the offing.
毫无疑问，将会有更多的变化。
she looks certain to win an Oscar.
她看来肯定会赢得奥斯卡奖。
■having or showing complete conviction about something
确信的，毫无疑问的
are you absolutely certain about this?.
这事你能绝对肯定吗？。
to my certain knowledge, I've never set eyes on her before.
我确信我从未见过她。
[attrib.]specific but not explicitly named or stated
某种的
he raised certain personal problems with me.
他问了我一些个人问题。
the exercise was causing him a certain amount of pain.
这种锻炼使他感到有些疼。
■used when mentioning the name of someone not known to the reader or hearer
某个的（用以提起一个读者或交谈对方不熟悉的姓名）
a certain General Percy captured the town.
一位叫珀西的将军占领了这座城镇。
pronoun