以aq0127的例句來說
第一句也可以說成, 不過兩者差不多
May I have an off day on Monday?
第二句的話由於是對上司講話,如果是在辦公室與上司面對面
應該改成
I would not be able to be here on Monday because of "你的理由".
I could not be here .....(同上)

對同事很簡單
I'll take a break on Monday
I have an off day on monday
I took a day off on Monday
I wll not be here on Monday(I'm not here on Monday)