After having done some research, here are my understandings of the uses of "the day of the week", but I am not sure I am right.
with a singular day of the week to refer to one occasion:
I’ve got to go to London on Friday.
with a plural day of the week to refer to repeated events:
The office is closed on Fridays. (every Friday) In informal situations, we often leave out on before plural days:
Do you work Saturdays?
Normally, "Monday", etc are nouns. However, In American English, "Monday" could be an adverb & "Monday (adv)=on Monday"
I'll ring you Monday
Similarly, "Mondays (adv)=on Mondays"
The restaurant is closed Mondays
a Monday (=one of the Mondays in the year)
My birthday’s on a Monday this year.
So, based on the 3 above rules, I would think these following sentences are correct:
I often do yoga on Mondays (repeated event)
I often do yoga Mondays (repeated event)
I will do yoga on Monday / next Monday (one occasion)
I went to Paris on Monday (one occasion)
However, I don't understand, why oxford dictionary has these example
On Monday(s) (= Every Monday) I do yoga.
On Monday I do yoga. (repeated event) ---> is this correct?
I always do yoga on a Monday. (repeated event) ---> is this correct?
I work Monday to Friday. (repeated event) ---> is this correct?