Something came up at MagicCon Chicago and there did not seem to be a consensus about. Posting here for further views.

Aidan controls the Celestus and it is day. He has a turn where a range of spells are cast back and forth during his end step (including at least two by him) and passes. Nóirín untaps and draws, plays a random spell, and attacks. At this point, they realise that it should have been turned over to night, and the Celestus would have triggered.

Day and night were being tracked by one of the double-sided day/night reminder cards, which was on Aidan's side of the table.

We considered which infraction might have been committed and how to resolve the point, and came up with three possibilities – there may be others.

1. GPE Game Rule Violation – as the players had not correctly changed the game to night, combined with a backup to the upkeep step where the violation occurred. However, no game rule requires a visible representation of the day/night state to be maintained, and this would potentially result in an inconsistent ruling between games depending on whether players chose to use a visual reminder of day/night.
   
2. TE Communication Policy Violation – in line with MTR 4.9, an erroneously-represented state should be handled as a CPV but with no warning. CPV allows us to backup “to the point of the action, not the erroneous communication”. “The action” here refers to the definition of CPV, which refers to when a player “has taken an in-game action or clearly chosen not to act based on the erroneous information”. Perhaps this allows us to back up to the upkeep step and put the trigger on the stack?
   
3. GPE Missed Trigger – on the principle that the game “knows” it's night even though the players failed to turn over a card, and as such Aidan's trigger was missed. No penalty as it's a beneficial trigger, and Nóirín chooses whether to put it on the stack.
   

None of these seem completely off-base so I thought I'd throw the question out there.

We have a policy update from Toby Elliot (came out with WOE) how we handle Day/Night: we can easy assess if it has become Night, because the previous turn's AP should have cast no spells. It is far harder to assess if it becomes Day if many turns have passed. We infract for CPV, downgrade it to no warning, and make it Night if we have no clear way to find out if it becomes Day or Night.

WOE policy update

Edited Kris Kleinsteuber (Feb. 29, 2024 12:10:28 PM)

I've typoed in the question. I should have said it was night and becoming day because of two or more spells being cast that turn.

We know it should be day, so with respect, the policy update you've linked doesn't assist.

The underlying philosophy is that no one person controls this status information and it can be easy to miss when nothing happens or a lot happens; we don't want to overly penalize players and hand out GLs if the base penalty was a warning.

Since the trigger was missed due to erroneous status information, it should be evaluated under CPV first, which allows us to back up the game to the point of the error. With the carved out niche for status information that is controlled by no one, we don't have to hand out a warning to either player. You second example seems to adhere to policy the best and allows us to fix the game state.