Savio Corvi reads:

While there is at least 1 Conspiracy card in play, Savio Corvi gains " Night . It is Night . Destroy all Day cads. Savio Corvi is immune to non- Day card effects.

There doesn't seem to be any timing or trigger for the "Destroy all Day cards" part of the ability to kick in. Usually Day or Night cards say something like "After xxx comes into play, destroy..."

Any thoughts on when this would happen, or how to resolve the logic via the FAQ or rules?

The way it is written 'Destroy all Day cards" is a passive effect. So any Day card that enters play is destroyed immediately right? I would imagine that was not the intention, but who knows. Probably needs clarification just in case.

That's what I was arguing with Kellen, but we think that is not the intention. Because then Day cards could not destroy Savio, right? With Passives resolving first and all?

I think it would depend on how the Day effect is written. For example the Agency support card that destroys all night cards, San Marco Basilica. The destroy all night cards is also a passive effect. Wouldn't the first player get to decide which passive resolves first? So it might not even be an issue unless your day card doesn't have a passive effect.

That's true, but that would mean that the opponent could not play "Daybreak", for example, in a defensive mode. So the non-active player would not have the opportunity to play an Event that has a "Day" characteristic.

I recognize this is a rare situation, but still, was the intention to prohibit this?

I think it's most likely a slight wording error. None of the other day/night destroy effects work like this to my knowledge. Probably something a mail to Damon will clear up?

Speaking of Savio Corvi and passives, it was interesting to realise that you can't use The Guardian conspiracy to search for Savio from your deck...

The Guardian (Terror In Venice F34) reads: "When you play this conspiracy, search your deck for a Day or a Night card, reveal it and add it to your hand. Then, shuffle your deck. While this conspiracy is in your won pile, increase the cost of any opponent's triggered effect that targets characters you control by 1."

I believe that The Guardian does enter play before it's effect kicks in, so there is now a conspiracy to activate Savio's "Night" trait were he also in play... But I assume while he's in your deck his passive ability is inactive, so he doesn't have the "Night" trait and cannot be located by The Guardian.

Official answers:

Savio Corvi's "Destroy all Day cards" ability is ongoing, and not a one-shot when he enters play like some other Day/Night cards.

If a Day card enters play that says "Destroy all Night cards", then there will be two conflicting passive effects between it and Saavio, and the active player (which will usually be the person playing the opposing card) gets to choose which passive activates first (so their Day card would take out Savio Corvi).

See the two posts here