Can somebody please explain why "du" is correct and "de" cannot be used?
If you said "la semaine nationale de livre", "livre" would have no article (and it needs one).
Du = de + le
....des livres. Accepted
"C'est la semaine du livre national" was rejected. I gave some thoughts on this. It is a national week, rather than a book week.
nationale (de) (le) livre
national of the book? wtf?
It is the national week to celebrate books, not the week to celebrate national books.