In Sunday afternoon vs. on Sunday afternoon
I have just taught my students about the prepositions of time: in, on, at."in the morning", "in the evening".and "on Sunday", "on Tuesday".

However, is it now "in Sunday afternoon" or "on Sunday afternoon".

I have googled, but it seems that even natives cannot come to a conlusion on this topic.

In my opinion: if I use "in" the preposition defines the part of day...in Sunday AFTERNOON and Sunday defines the afternoon further. Using on...the prep. goes to the day: on SUNDAY afternoon.

However, anyone here, who can enlighten me?

Question

In Sunday afternoon vs. on Sunday afternoon
I have just taught my students about the prepositions of time: in, on, at."in the morning", "in the evening".and "on Sunday", "on Tuesday".

However, is it now "in Sunday afternoon" or "on Sunday afternoon".

I have googled, but it seems that even natives cannot come to a conlusion on this topic.

In my opinion: if I use "in" the preposition defines the part of day...in Sunday AFTERNOON and Sunday defines the afternoon further. Using on...the prep. goes to the day: on SUNDAY afternoon.

However, anyone here, who can enlighten me?

Answer

It's always "on" with days in the examples you give, regardless of if they are qualified by time.  "On Sunday afternoon" is correct.   A better example for what you are describing, where the preposition changes, would be "In Sunday's class" or "in Sunday's football match", where the preposition becomes 'in' because it is referring primarily to the class/match not the day.

Answer

Could you please give some examples of instances you have come across of 'in Sunday afternoon' ? As far as I am aware, there is no 'topic' and no lack of conclusion - the only possible form is 'on'. I'd be interested to know where these examples come from.