a day of, or the day of?

Question

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.

Edit:

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.

Answer 1

Whether to use "a" or "the" depends on what you are trying to say.
If you habitually rest on a particular day, then you would use

the day of rest

and refers to a particular day one does not work and can be understood to be naming that day as a "day of rest"

a day of rest

refers to a day, in general, when one does not work

"A day of rest" is not necessarily "the day of rest", but
"the day of rest" is definitely "a day of rest".

In your book analogy sentence sounds a bit awkward

I like to write stuff on the book when I annotate a book.

You are actually using the shorthand of

I like to write stuff on the book (that I am annotating) when I annotate a book.

and the meaning is

I like to write stuff on the (cover of the) book that I am annotating.

Using "on" implies you are writing on the outside of the book in your context. If you were writing regarding the contents of the book, you might use "about"

I like to write about the book (I am annotating) on its cover.

Answer 2

Your sentence provides details about why you can finally relax, not about day of rest.

As such, it is most natural to use a day of rest, as you are not talking about any day of rest in particular. You would use the day of rest if (a) you assume your hearer can identify which day of rest it is; or (b) if you want to make day of rest the topic of the discourse (and so you will go on to talk about the day of rest).

You have to more tightly link these reasons/details to day of rest in order to make it definite. One way to do this is by using the day of rest to head a relative clause that further defines it. Consider any of the following:

"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 the day of rest I had long been looking forward to."

or

"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 the day of rest I deserved after working so hard."

Now, the hearer can identify which day of rest you are talking about. And only now are you being "very definite" about which day of rest you are talking about.