The National Library of India is the largest library in India and one of the largest libraries in the world. It is located in New Delhi and was established in 1891. The library is open to the public and provides access to a wide range of books, periodicals, manuscripts, maps, newspapers, audio-visual materials, and other resources. The library also serves as a repository for all books published in India. The term 'National Library' is derived from the fact that it is the national library of India. The answer to the crossword clue is 'WEEK', which refers to the library's opening hours, which are seven days a WEEK.