On ~ afternoon implies that the afternoon is a single point in time; In any event, the impressive rise of free of against free from over the past 100 years suggests that the english-speaking world has become more receptive to using free of in place of free from during that period. In ~ afternoon suggests that the afternoon is. · what is the opposite of free as in free of charge (when we speak about prices)? · if so, my analysis amounts to a rule in search of actual usage—a prescription rather than a description. · 6 for free is an informal phrase used to mean without cost or payment. these professionals were giving their time for free. We can add not for negation, but i am looking for a single word. · the choice of prepositions depends upon the temporal context in which youre speaking. The phrase is correct; You should not use it where you are supposed to only use a formal sentence, but that doesnt make a phrase not correct. Thus, that temporal context would take the entire afternoon as one of several different afternoons, or in other words, one would use on when speaking within the context of an entire week.