I think it should rather be on each episode
and the system should automatically input the "currently on" time in each episode when it is created, which the contributor could manually override.
The problem is, as Chris wrote, day and airtimes change over the course of a show's life or even during a single season! So, just looking at the "currently on" time would give someone the impression that each and every episode aired on that day and at that time when the airday/time changed two months ago... so, some of them did and some of them didn't. I think it is impractical to rely on contributors to go back and manually input the previous time for all prior episodes each time the "currently on" time changes.
Now, keeping track of changes in day and time schedules, as far as overall show history
("well, it used to air on Saturday night at 10:00 following Matlock
, but then it changed to Fridays at 8:30 which put it head-to-head with Night Rider
"), should be done in the season guides in my opinion.