There are two common ways to show that a noun is numbered, i.e., that it occupies a particular number in a list of things of the same type. Those two ways are:
- using an ordinal number before the noun
- adding the number right after the noun, optionally preceded by the word number
Ordinal numbers (first, second, third, etc.) function more or less like adjectives, and as such, they can be used as part of both definite and indefinite noun phrases—the difference being whether the noun phrase as a whole should be considered definite or not in the context it appears.
Since ordinal numbers are most commonly used to limit a range of things down to one specific one (the one that occupies the nth place in the list), which makes the noun phrase definite, the definite article is much more common than the indefinite article.
The other way to express a certain number in a list or range functions differently. The number (with or without the word “number” itself) added after the noun does not act as an adjective as such, though they are quite similar to; what exactly it is generally categorised as syntactically, I don't know offhand.
Crucially, though, this postpositive number inherently marks the noun phrase as definite. In that sense, it functions like Saxon and pronominal genitives.
Saxon genitives act like determiners (like articles) and mark the noun phrase as definite—but since you can't have multiple determiners and you can't mark a noun phrase for definiteness more than once, you cannot combine a Saxon genitive with an article: *my the house doesn't work.
Postpositive numerals are not, as far as I know, generally considered determiners, but they do have the effect of marking a noun phrase as definite and blocking at least articles: *a week 33 doesn't work, nor does *the week 33.
Oddly enough, they don't block other determiners like Saxon and pronominal genitives: My week 33 is looking terribly busy is a bit awkward and not how most people would phrase it, but it's grammatical. A more likely scenario would be a student in class who, upon being told to flip to page 20 in the class book, raises his hand and says, “But my page 20 has been torn out!” (meaning “page 20 in my book”).