I ran into the same thing earlier this week trying to book a YYC-YHZ flight on the 23rd. Ended up on an ugly 12-hour YYC-YUL-YFC-YHZ odyssey but I'm hoping to get on a better routing via a
same day confirmed change.
I suspect the issue is a combination of the end of reading week for university students and a popular week for family vacations with most provinces having family day on the previous Monday.