grammar - "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.

Answer

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.

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

For example:

I work out Monday mornings.

See you Friday!