When it's not per policy.
LOL. The DAY OFF box is to be marked, and OT paid, when you're working a regularly scheduled day off during 5 day workweek and will be working 6 days (or a day off during a 4x10 workweek and will be working 5 days). Your belief is that payroll wouldn't give it to you if you weren't rightfully entitled to it, and that's laughable. There's a line of people a mile long who've had to pay money back to payroll because they were paid money that wasn't rightfully theirs, me included. You're lucky you didn't get caught.
Again, when pressed for a policy citation for something you think is policy, you don't have one because you don't know what you're talking about.