In years past, day-times were staffed by whoever was in town and nights were staffed by people who could not leave work during the day or were out of town.
There were regular, assigned night time crews who always rode together and sometimes were just husband and wife. No scheduling was needed because they had assigned times and swapped with other crews as needed. There were crews for all nights except Saturday and Sunday because no one wants every Saturday tied up. So a system was evolved in which "Saturday duty" rotated among the 5 other crews once every 5 weeks. Saturday duty covered from 6PM Saturday to 6AM Monday. Later, a Sunday night crew was established and the 6 crews rotated once every 6 weeks and weekend duty was reduced to 6PM Saturday to 6PM Sunday.
Our calendars still reflect this. The so-called night-weekend calendar covers every night 6PM - 6AM (12 hour) plus Saturday 6PM to Sunday 6PM (24 hour)
The day-time calendar covers all day-time hours not on the night-weekend calendar, that is, Mon-Sat 6AM-6PM.
There are two different software presentations. One for day and one for night. The day-time software was written first before we started scheduling nights and when I wrote the night-time software I thought a graphical presentation would be nicer. I tried to get one calendar and one software for both day and night accepted, but the Squad resisted so we have two.