"The seventh day"
Without any context, it's not a direct object. I skipped over the lesson that taught the accusative case, but it obviously did it really badly because lots of people are having issues confusing definite with accusative. A lot of people seem to think the accusative case just indicates "the". It doesn't.
In English, when we use "the", it indicates that the noun is "definite", it refers to something that the speaker expects the listener to be able to identify. If I say "the moon", I expect you'll think of the Earth's moon and not one of the moons of Saturn. If I tell you I bought a book and then want to tell you more about the book, I'll call it "the book" so you know it's the same one I've already mentioned.
The direct object (often marked in languages by an accusative case) is the person or thing in a sentence which the verbs action is performed on. For example "The man [subject - nominative] hits [verb] the dog [direct object - accusative]."
Definiteness is not marked reliably and simply in Turkish. There is nothing that really corresponds to "the".
There is also nothing which simply marks the direct object of a sentence. What is called the accusative case in Turkish is only used for definite direct objects.
Indefinite subject: Nominative (unmarked)
Definite subject: Nominative (unmarked)
Indefinite object: Nominative (unmarked)
Definite object: Accusative (with -(y)I)
Adam köpek dövdü. = A/the man hit a dog. In this sentence, the subject could be indefinite (a man) or definite (the man), but we know that the object is indefinite (a dog) because it is unmarked.
Adam köpeği dövdü. = A/the man hit the dog. Again, we don't know if it's "a man" or "the man" because there's nothing to tell us this in this sentence ... but since the dog is the object and it is marked, we know it's "the dog".
If you had to translate "The man hit a dog" and wrote Adamı köpek dövdü, thinking that the accusative simply marks "the", what you have actually said is "the dog hit the man". You've reversed the roles in the sentence. (Because of the unsual word order in that sentence though, there is emphasis on the dog, so it's more like "It was the dog who hit the man." To say "the dog hit the man" without any special emphasis would be Köpek adamı dövdü)
Just to compare with another language ... In German, you can mark all four combinations. Both man and dog are masculine in German, so in the nominative, "the" is "der" and "a" is "ein". In the accusative case, "the" is "den" and "a" is "einen".
Der Mann schlägt den Hund. (the man, the dog) Ein Mann schlägt einen Hund. (a man, a dog)
In German, you can see that nominative and accusative do not depend at all on whether the noun is "the X" or "a X". Turkish is fairly unusual (but not unique) in having the accusative case only marked when it's definite ... and this leads learners to somehow think that the word "the" means the noun should be accusative.
So ... to come back to this "sentence", if you just say "the seventh day" on its own, the default is nominative. It will be nominative in a sentence like Yedinci gün çok güzeldi (The seventh day was very beautiful) but accusative in a sentence like Yedinci günü hatırlıyorum (I remember the seventh day) because it's the direct object.
(Turks, please let me know if my examples are weird. I wasn't sure if "dövmek" was the right verb, and maybe my discussion of the emphasis is a bit off.)