Some purists object to the use of these plural forms in the senses of "during the afternoon" and "every afternoon." Idiomatically, however, it is permissible and correct to say or write "Sue works only afternoons" when what is meant is "Sue works every afternoon" or "Sue works only in (or during) the afternoon."
In various parts of the country, afternoon, evening, and morning have different meanings and applications. Afternoon means the time from noon until evening, but when does evening come? In certain sections of the West and South, one hears "Good evening'' as a greeting anytime from noon until midnight. To some, evening may mean the period between noon and nightfall. But what is night? Is it the time between sunset and sunrise? Does morning apply to the time between midnight and noon or that between sunrise and noon? The only sensible plan is to use these terms as your friends and neighbors do, recognizing that such usage may be misunderstood in other sections of the country.
Dictionary of problem words and expressions. Harry Shaw. 1975.