According toIn accordance its grammatical relations within this particular sentence, it is perfectly possible, therefore, that weekdays is a noun heading its own Noun Phrase.
Its internal morphology suggests it is a noun.
ItThe phrases it heads can fulfill the syntactic functions that are typical of temporal nounsnoun phrases: Subject, Object, Predicative Complement and Adjunct.
As with other common nouns, it can function as a modifier in a noun phrase.
It can be modified by other nouns and by adjectives.
It cannot be modified by adverbs.
It has both a singular form and a regular plural form.
It has a genitively inflected form.
It can occur with a large range of determiners, most importantly with the determiner every.