Hi, Brahman! In my country (Uzbekistan) English is taught as a foreign language.
As we know, there is an expression in the morning and in the mornings
In the same way
In the afternoon & in the afternoons
In the evening & in the evenings
But if there is an adjective before these expressions, as far as I know,
the preposition “in” should be normally turned into “on”
On Monday evening(s)
On Friday afternoons(s)
On Sunday evening(s)
For my money (<-- it’s an idiom that I learnt on Englishclub.com),
Selina Hastings has said “on’...
I hope this answers your query!