Vadrik, Astral Archmage

If it's neither day nor night, it becomes day as Vadrik, Astral Archmage enters.
Instant and sorcery spells you cast cost {X} less to cast, where X is Vadrik's power.
Whenever day becomes night or night becomes day, put a +1/+1 counter on Vadrik.

Innistrad: Double Feature

Innistrad: Midnight Hunt

Innistrad: Midnight Hunt Promos

 Before a player untaps their permanents during the untap step, the game checks to see if the day/night designation should change.

 Day and night are designations that the game itself can have. The game starts as neither. Once the game becomes day (or less commonly, night), the game will be exactly one of them—day or night—going back and forth for the rest of the game.

 For more information about Day and Night and the Daybound and Nightbound abilities, please see the Innistrad: Midnight Hunt Mechanics atricle (https://magic.wizards.com/en/articles/archive/feature/innistrad-midnight-hunt-mechanics-2021-09-02).

 If it is day, and the active player of the previous turn cast no spells during their turn, it becomes night.

 If it is night, and the active player of the previous turn cast two or more spells during their turn, it becomes day.

All costs to cast a spell are locked in before mana abilities are activated and those costs are paid. If Vadrik's power changes after costs are locked in, or if Vadrik leaves the battlefield while you are activating mana abilities or paying those costs, the cost reduction effect won't be affected and the total cost to cast the spell won't change.

If a triggered ability triggers whenever day becomes night or night becomes day, it only triggers if it was already either day/night and becomes the other. It does not trigger when the game becomes day or night for the first time.