Questions & Answers about La biblioteca abre de lunes a viernes.
Why does the sentence use biblioteca instead of librería?
The word biblioteca means library, which is the place where you borrow books. The word librería is a false friend; it looks like library but actually means bookstore.
Why are the words de and a used here instead of desde and hasta?
Why aren't the days of the week lunes and viernes capitalized?
Since lunes and viernes end in an -s, are they plural here?
No, the Spanish words for Monday through Friday naturally end in an -s even in their singular form. To tell if a day is singular or plural in other sentences, you have to look at the article used before it, such as el lunes for Monday or los lunes for Mondays.