I thought of another question led by a question in an exam.
The paper is due next month, and I am working seven days ______ week, often long into______night.
A． a; the
I agree with the correct answer A, but it brings me to a new question whether it is appropriate to write the sentence as "The paper is due next month, and I am working seven days a week, often long into /(the zero article) night." Or maybe it is good to put the question in this way, which is what the difference is between "into night" and "into the night".