There is nothing definite. I have seen "next day" and "the next day" both used. I don't believe it is absolutely necessary to use "the" in this case. However, "the following morning" is the only form you can use. If you use "the next day", you can't go wrong. It will never be considered wrong, with one exception: If you use it as an adjective, don't use "the". Example: We sent the letter by next-day service. (Notice the hyphen '-') Also note the "next week", "next month" and "next year" generally don't require "the" are are used often. As to why this is so, I think it's just one of those funny things that develops in a language where no one can really say who it cam about.

In the United States, "next day" is rarely used this way, and then usually in writing only. I wonder if your textbook was printed in the United States or in Great Britain. It seems to me that the use of "next day" as an adverbial phrase may be a British usage.

Hello Joseph, I would like to comment on fdmaxey's answer. He said *quoting him* : "If you use "the next day", you can't go wrong". I agree with him in general but particularly in the example you mentioned it wouldn't be appropriate to use " the next day". Refraining from using the definite article "the" in this sentence is for a good reason. Making 'next day' indefinite refers to the fact that the decision the woman took in her anger to carry out a certain action of punishing the assistant is not yet put in effect. In the sentence the next day is still to come and her intended plan is still uncertain. Another thing is that the "day" is an indefinite period of time in contrast to the morning,that is the time period between dawn and noon.The indefinite "next day" opposed by the definite morning has as a meaning, that the exact time of carrying out her action in that day was not determined previously. Whether she was going in the morning , noon or afternoon wasn't yet clear. In the next sentence " the following morning" the action took place at a definite time. "That is what real English is " using in certain cases words or expressions that have direct or indirect implications that are sometimes subtle to detect or analyze. That doesn't mean that using some words or omitting others will not have its impact on the overall meaning of the sentence.

I also learn from the Concept English,but I dont read so carefully like you^-^ By the way,the Concept English is really useful to me. fdmaxey`s answer is good,and I`ve got it.I think that it is a sense of language,just like Chinese,there is no clear rule about some sentences:)