I found that somebody did a bunch of research and posted this on last year's thread, it's the dates/days in March that the decisions came out:
2021 10th - Wednesday
2020: 11th - Wednesday
2019: 5th - Tuesday
2018: 6th - Tuesday
2017: 8th - Wednesday
2016: 8th - Tuesday
2015: 3rd - Tuesday
I also found that last year (2022), they came out on Wednesday the 9th.
Here's the IS waitlist movement from 2011-2018.
Last year, it moved about 30 spots for IS students.
There was one year, I believe 2019, that it only moved 12 spots.
Note, this is IS only.