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, negotiation, and fusion, and often 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.

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

Digital Devil Saga: Avatar Tuner
The moon phases are replaced with the "Solar Noise", which has the same basic function as the moon phases.

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. Certain abilities recieve a damage boost during certain moon phases, as well.

Persona 4
Weather takes the place of the moon phases, with different weather creating a different effect. Important events will occur during foggy days. The effects of the moon phase on fusion are effectively replaced by the Fusion Forecast system.

Shin Megami Tensei: Strange Journey
The Schwartzwelt's moon serves as the Moon Phase System. Fusions are more likely to fail in full moons, demons act irrationally and relax their inhibitions allowing for recruitment of certain races (e.g. Tyrant, Foul), and certain attacks work better on certain phases.