[HMM III OFF TOURNAMENT] "Race for the Leader"

Question for the organizer.
For example: I take the last castle from the AI, skip my turn, and the AI dies (either by attacking or after 7 days, it doesn't matter). It's month X, week Y, day Z. The question is, on which day is the victory counted: day Z or Z+1?

So, the AI dies during the turn transition, is that correct? If so, you have a save from day Z, when the AI is still alive. Therefore, if you click "next turn" and the AI dies, you will be credited with Z+1 day.
Therefore, it is not in your best interest for the AI to die on its own during the turn transition. It's better to play out that day Z+1 yourself, defeat the AI, and try to improve your skills and collect more mercury. ;)

Good luck!