The sales department may think about seasons... not radio seasons.... but retailer seasons. Valentines candy and other merchandise is a season. Summer is a season for car-repair-before-your-vacation advertising, etc. etc.
If there are any "seasons" in radio it would be the "ratings seasons". Audience measurements are usually scheduled at certain times. (They do not take place 52 weeks per year.) You can pretty well be that if a radio host if absent... as in "on vation"... it is NOT ratings season.