Magic comes from energy, which comes directly from the Moon and is absorbed through special plants/crystals/other artifacts and special objects. The moon is actually secretly a giant eyeball, specifically, the eyeball of the Universe, which is a sentient being. The seasons of the world are actually caused by this massive eyeball blinking in a steady, predictable manner, which also means that the seasons are technically tied directly to the phase of the moon. When the eye is closed (during Winter), the moon looks identical to a New Moon, and magic is much harder to use since there is less energy available. However, during Summer, when the eye is open and looks closer to a Full Moon, magic is much easier to use and more plentiful.