Symbolism, Imagery, Allegory
No matter where you are from, the sun is an omnipresent entity that we all rely on, right? But how it is seen can change from location to location. If you live in Tucson, Arizona the sun is seen as a cruel, merciless, scorching enemy to be avoided at all costs if you value your skin or hydration. If you're from Seattle, Washington the sun is something that only pokes its head out on rare occasions and thus needs to be worshipped accordingly. If you're from anywhere in New England the sun is a fickle fellow that packs a ton of heat in the summer and little warmth in the winter. In the Glade, however, the sun represents hope and normalcy. Wherever they are, at least it has a sun that rises in the morning and sets at night. Well, until the Ending is triggered and the sun totally disappears. When the boys are left with a gray monotonous "sky" overhead they experience the loss of hope:
In other words, the sun that had shone down on these people for two years, providing heat and life to everything, was not the sun at all. Somehow, it had been fake. Everything about this place was fake. (34.11)
We can only imagine how bleak it must feel to have the one source of warmth and light and normalcy in your life to disappear.