Big Babooba has a point. Back when I started in the Atlanta Hub, it was Monday through Friday, 11 PM till 3 AM. Some buildings do have a Sunday night sort.
Our Midnight Sort at Denver runs Monday through Friday. Its my understanding that years ago, it ran Sunday-Thursday. Now, the Noonday Sort runs Sunday-Thursday. One of the reasons for the change was the volume didn't justify running the Night Sort's staffing on Sunday. The change was probably made right after Denver International Airport opened in the mid '90s. The purpose of having a sort run on Sundays is to keep volume--particularly air--moving. This was the way it was explained to me right after I was hired in January. -Rocky