Something that’s overdue should have happened or been dealt with a long time ago, like the long overdue recognition for the people who rescued motorists in last winter’s big snowstorm.
Overdue literally means “past the due date.” Bills that aren’t paid on time are overdue. So are library books not returned or renewed by the date specified by the library. However, some things that are overdue don’t have a particular date associated with them. Instead, it is up to subjective perception: if it seems “too long” then something may be considered overdue, like an overdue visit from a friend or a positive change that’s overdue.