核心问题
Monday evening he turned up at the flat.
Monday evening 是否应改为 On Monday evening?
词典解答
在 evening 之前有 Sunday, Monday... 作定语时, 其前的介词 on 在非正式语体中是可以省略的, 见 on Sunday evening/Sunday evening。
of an evening 见 of (of an evening)
( on ) Sunday evening 见 in the morning/on the morning