Vadrik, Astral Archmage

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.

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.