The selection of prepositions relies upon upon the temporal context through which you are talking. “On ~ afternoon” implies that the afternoon is a single time limit; thus, that temporal context would take all the afternoon as considered one of a number of completely different afternoons, or in different phrases, one would use “on” when talking throughout the context of a complete week.

“In ~ afternoon” means that the afternoon is a temporal house in-and-of-itself, whereby something that occurs will occur amongst many different occasions. In different phrases, the temporal context for this utilization could be if one have been talking of a single day — whether or not previous, current, or future — and of a single afternoon, throughout which many issues may occur.

Consider it like this:

“She referred to as me Saturday morning, and mentioned she’d give me her reply within the afternoon.”

Within the above instance right here, one can solely use “in”, and never “on”. Equally, one makes use of “in” within the following instance, as nicely:

“She’s going to name early Saturday morning to examine in, and can give me her ultimate reply within the afternoon.”

Within the following instance, although, one should use “on”:

“She referred to as me yesterday afternoon, and mentioned her mornings are too busy to speak. She’s nonetheless unsure what her plans are for Sunday, so she’ll solely be capable to give me her reply on Saturday afternoon.”

Because the above commentator suggests, one can by no means say “within the Saturday afternoon” — however i believe you already know that. In any occasion, from the above two examples i believe it is clear that the selection of “within the afternoon” versus “on Saturday afternoon” is dependent upon the temporal body of reference, and the context through which you are talking.