"Weekdays" used as an adverb

I found a sentence in Oxford Advanced Learner’s Dictionary:

open weekdays from 9 a.m. to 6 p.m.

The bookstore opens weekdays from 9 p.m. to 6 p.m. .

How do we understand the structure of this sentence? I know it means something is open on weekdays from 9 a.m. to 6 p.m.

In your reference, you can see that the word "weekdays" can be used either as a Noun (which you normally know) or as an Adverb:

The centre is open on weekdays. (Noun)

The centre is open weekdays. (Adverb)

In the second example, the adverb is describing the verb. So it doesn't need a preposition anymore to connect to the rest of the sentence.

In addition, we sometimes drop "on" before days in spoken English.

For example:

I work out Monday mornings.

See you Friday!

I believe that the full sentence is "open on weekdays from 9 a.m. to 6 p.m.", and the "on" was dropped somewhere along the way. So "weekdays" here isn't actually used as an adverb, it just seems that way.