Ah, just in case you're not setting up a joke, the light refers to the elemental magic of light, one of the six elemental forces (light, dark, fire, water, wind, earth). It's more like an energy with properties including that of light, than actual light emitted by the sun and such. 'Eternal' refers to the close proximity to the apex of an element, apex here meaning an immense pool of the given element in nature. Such that it will resonate to surrounding area in some ways, which isn't necessarily visible and can be as simple as making it easier for sorcerers to channel and replenish their magical capability just being within miles of the apex. The day and night come as normal, just as fire and water can exist without the magical part of it.