Moon Phase System

The Moon Phase System is a component of a vast majority of Megami Tensei games. The various phases of the moon often affect battle and fusion, and may affect other aspects of the game as well. The moon phase changes as steps are taken, and it often functions as a counter for item and spell effects, such as a spell lasting until the next new moon.

In Shin Megami Tensei III
In Shin Megami Tensei III: Nocturne, the moon no longer exists. Instead, the phases of Kagutsuchi play almost the same role the moon did before.

In Persona 3
In Persona 3, the moon phases play similar roles as in other Megaten games, except that the moon phase is determined by days of the month as opposed to footsteps. Every time the full moon appeared, it would signal the appearance of a strong Shadow.

In Persona 4
In Persona 4, weather takes the place of the moon phases, with different weather creating a different effect. According to scans from Famitsu, important events will take place on misty days.