Everyone knows we use “in” for these terms
In the evening, In the morning, in the afternoon.
And for days, we use the preposition “on”, on Saturday, on Monday, on Tuesday,…so on.
With respect to both of them being “nouns”, Sunday and evening.
But I am confused of using them together.
- I met one of my friends on Sunday evening.
Why do we use ON instead of:
I met one of my friends in the evening of Sunday.
I met one of my friends in Sunday evening
Here, in the last sentence, I used IN Sunday evening, as we do with “On school vocation”; ON related to vocation, not school. So here IN related to evening, not Sunday.
Best Answer
Use of prepositions in English is not a case of "why", but rather a case of "because" that is the way we do it.
Generally, we use "on" with specific days, or dates. Sunday evening is a specific day. If we were to use "the evening" on its own, it could refer to any evening, and is not therefore a specific day or date.
I really do wish I could tell you "why" we use on, but it would seem to be a rather arbitrary choice, with no particular logic to it.
More info here: In, at, on + Time or Date
It might be pertinent to note that other language use different ways of expressing the same ideas. In French for example, prepositions are not used at all with days of the week.