Testing day before release makes assumption there are no issues to fix.
Our team got around it by adding +50% time to sprint for fixes or adjustments. If there are no issues to fix, we have this already booked time to deal with tech debt or fix punch of smaller issues that never get highest priority on their own.
In our case what happens is that the "release day" is not a fixed day because there was too much to test and this is a problem that speaks for itself :D
I'll work on changing the approach!
We struggled too, as our tester was part of another department, hardly ever available. Once we managed to explain management it's a bottle neck, and it didn't happen over night, eventually we got dedicated tester for our team. Think it was around two years ago. It has made a massive difference, we get our feedback very quickly, no slipping deadlines, everyone's happy :)
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