this day 指的是当今,相当于today;可以指当下时代,也可以指说话当天。具体要看上下文的意思。
on this day指说话时所指的当天。有强调意味。如:I'm too tired! I do not know when to come on this day!
英语当中不用介词是简化语言表达语句的需要,只要不产生歧义,能简则简。有时为了平衡句子的需要,需要介词,特别是后面有修饰语时,如:We must do it on this day which is the last for us to use.
另外,in this day,指在某一时代,也用in sb's day.
this day是指这一天的日期(我认为是)
on this day 是指在这天中
.....
this day 指这天(e.g. this day was very sunny)
on this day 指在这天内(e.g. I did nothing but eating and sleeping on this day)
this day是指今天
on this day是指这天