Good question, and a difficult one. The difference is in how the time reference is working. You are right that normally we say "In the afternoon" when we are indicating that it was "during the afternoon" that it happened (as opposed to in the morning or in the evening). However, when we are indicating the day on which something happened, we use "on". It happened "on Friday". Where the confusion comes is when we are indicating both the day and the time of day. The day takes priority. So we say "On Friday afternoon." Your example sentence actually works this way. Although it doesn't mention the day by name, it indicates that it is talking about a particular day, the day of the dinner. So an expanded version of your sentence would say "On the day of the dinner during the afternoon." When you shorten it to "on the afternoon of the dinner" the same rules apply. If that is still confusing, here is another way to think about it. When it specifies "the afternoon of the dinner", what is it separating it from or comparing it to? Is it separating it from "the afternoons" of other days, or is it separating it from "the morning or the evening of the day of the dinner"? When the alternatives are the other days of the week, you will use "on" - "Not on Friday afternoon, or Sunday afternoon, but on the afternoon of the dinner." When the alternatives are the other times of the day, you will use "in" - "In the afternoon on the day of the dinner" or "In the afternoon of the day of the dinner" Sometimes questions that seem simple are the most difficult to answer in a simple way!

This is a good question. We generally use 'on' when talking about a specific day. 'I went swimming on Saturday morning.' 'In' is used when the day is already known to us. 'I'm going swimming on Saturday.' 'When?' 'In the afternoon.' 'On the afternoon of the dinner' is right because it is the afternoon of the day of the dinner. It's a specific day (the day of the dinner) and I assume the day has not been mentioned in the sentence or paragraph beforehand? EDIT. Thinking about it, even if you knew the dinner was to take place on a certain day (let's say Tuesday) you would still say 'on the afternoon of', I'm not 100% sure why. 'In the afternoon of' in this context sounds wrong.

Nathan is right. We use "on" for days and "in" for parts of days. However, when we specify both the day and the part of the day, the day takes precedence, so we use "on". Here are some examples: on Friday morning on Monday afternoon on the morning of the exam on the afternoon of the match NB. This is standard English and it applies to all varieties. Both British and American English speakers follow the same conventions.

You have discovered a difference between American and British English, but in your sentence what Ray and Ellis have said is correct. I think ! that the same answer will come from American speakers regarding this sentence. We will have to wait and see. Before the weekend arrives. on the weekend AMERICAN and at the weekend BRITISH is something both nations agree to differ over.

they are talking about the particular day at some point in the afternoon