I wrote this thing;
"I woke up as any other usual day, but my spirit was high. After a long period of non-stop, vigorous working, I finally could relax, as it was a/the day of rest."
I strongly feel that "the" can be used in that sentence, as it is a reason, a very definite reason that is, for my being able to relax. Therefore, even though my readers will not have any information prior to reading this, I think "the" should be used.
Am I right? I always have difficulty knowing which article to use. If I am wrong, please tell me the difference between the nuances of those uses.
I think that implied noun is "the day of rest that finally allowed me to relax", which I think is a referent unique enough to be used with a definite article.
Like this: I like to write stuff on the book when I annotate a book. It is sure that I am not referring to a specific book. But I used "the" instead of "a", because it refers to a book that I am annotating in a hypothetical situation. It is implied, which I think makes it definite.
Please correct my analogy if I am wrong.