Lamp

What is night?

This is a game state that changes via card effects. Being "night" does not inherently trigger any rules by itself, but certain cards have abilities that activate or become active while it is night. Additionally, if either player changes the state to night, the abilities of both players that become active while it is night become active for that duration.

I have multiple copies of the same card with a [Once Per Turn] ability in my base. Can the [Once Per Turn] ability be activated once for each individual card?

Yes, it can be activated. For example, if you have multiple copies of TD01-008 〈Stone Pit〉 in your base, even if you activate the 【ACT】 ability of 〈Stone Pit〉 A, you can still activate the 【ACT】 ability of 〈Stone Pit〉 B.

I have BP01-073 《Helzephyr – Wings of the Moonless Night》 and BP01-088 《Lamp》 in my base, and the game state is night. If one of my Pals that does not have 〈Nocturnal〉 is deployed, can I put a Pal into the graveyard using the 【AUTO】 ability of 《Helzephyr – Wings of the Moonless Night》?

Yes, you can put it into the graveyard. Even if a Pal originally lacks 〈Nocturnal〉, it gains 〈Nocturnal〉 through the ability of 《Lamp》, allowing you to put a Pal into the graveyard with the 【AUTO】 ability of 《Helzephyr – Wings of the Moonless Night》.

On my turn, I played an ability that states 〈It becomes night until the end of the opponent's next turn.〉 If the card that played this ability is put into the graveyard during the opponent's next turn, does it stop being night?

No, it does not stop being night. Even if the card that played this ability leaves the base, it is night until the end of the opponent's next turn.