He's tied to the moon, specifically any moon phase with an orange-red glow from smoke and other particles in the air. He's the most corporeal during the harvest season. When the moon is over the layer of particles that causes it to appear red and orange, he will fade out of this pane once again. The more of the moon that is visible, the stronger he is, as well. His sister Feidlmid is tied to the new moon, and can partially present herself if there is a "halo" around it in the area she wishes to be.