I agree with Claire.

You can and should. On Monday morning - one time this Monday or last Monday. On Monday mornings - every Monday. But these days, Monday morning is often used to mean every Monday too.

"We always have meetings on Monday morning." "On Monday mornings we always have meetings" "Every week we have meetings on Monday morning." All of these sentences work. The last sentence is more formal.

We always have meetings on Monday mornings. Keep things plural with plural.