Should Dark Day boost the power of certain spells? Should there be a corresponding Day of Light?
Perhaps the addition of Relah as a moon has accentuated or amplified the destructive phase of the Ethereal Plane. Perhaps Dark Day serves to increase lunar worship.
Day of Light could be an effect by Asaliah and Tiriel to emphasize the creative phase of the Ethereal Plane as a counter to Relah's Dark Day. Thus, Dark Day and Days of Light are physical manifestations of this theological struggle at the deity level.
Spells with the Destruction, Shadow or Darkness descriptors operate as if affected by the Maximize Spell, Extended Spell, or Persistent Spell feats (as appropriate) during Dark Day. Spells with the Creation or Light descriptors are similarly enhanced during Days of Light.
Magical light sources have their areas of illumination decreased by 50% during Dark Day and increased by two-thirds during a Day of Light. Mundane light sources have their areas of illumination decreased by two-thirds during Dark Day and increased by 50% during a Day of Light.