Do I break a 10 day fast at dinner (day 10) or in the morning (day 11)?
If you consider fast to be dinner to dinner than OMAD would be 1 day fast every day. And that’s not what people think when OMAD comes to mind.
Considering circadian cycle and slow digestion over night your fasting metabolically started in the morning when you skipped the first meal.
So when you don’t eat for an entire circadian cycle from waking to sleeping again would be one day that you fasted.
Not that this matters as you can fast as much as you want.
I think it depends on what you are doing the fast for/preference. I prefer to break at night, because that is when I am hungriest and it helps me sleep, also I am least hungry in the morning, and usually feel like I could keep going without using much willpower then. But you can run into bathroom issues after breaking a fast that long, I know some people prefer to be awake to deal with that, so that might be a benefit to breaking it in the morning.