I met them on ____ morning last month.

[tom3m]

I met them on ____ morning last month. a) Monday b) a Monday c) the Monday

I would like to know whether we can write days with articles. So far, I have been convinced that days are only used with a zero article e.g. They are not working on Monday.
If we can, the difference of form must inevitably imply a difference in meaning. What exactly is different?

Thank you in advance

[5jj]

I met him on a Monday morning last month. - We don't know which of the four or five Mondays in that month.

I met him on the Monday morning last month.
I met him on Monday morning last month.

I can't at the moment think of completely natural contexts for these.

[Tdol]

I can't at the moment think of completely natural contexts for these.

Not with those time periods, but if we change them we can do it easily enough:

I was there for the whole week and saw them on the Friday.

[emsr2d2]

I don't work on a Monday = I don't work Mondays.

Choice a) in your original example works exactly as 5jj described.

Q: Have you met James before?
A: Yes, we met in London. I can't remember when exactly but it was a Saturday.

[tom3m]

Thank you,
I have , unfortunately, never come across this usage of articles before days. Is there a thorough article/explanation to this you can recommend?

[bhaisahab]

Thank you,
I have , unfortunately, never come across this usage of articles before days. Is there a thorough article/explanation to this you can recommend?

"Monday" is a countable noun. (As are Tuesday, Wednesday, Thursday etcetera.)

[Tdol]

I saw him on Monday. - The most recent Monday.
I saw him on a Monday. - One Monday from a number (last year, for instance)
I saw him on the Monday. - A specific Monday known to the listener (the week I was in London, for instance)