Mainly to avoid all the staring eyes, Harry looked upward and saw a velvety black ceiling dotted with stars. He heard Hermione whisper, "It's bewitched to look like the sky outside. I read about it in Hogwarts: A History."
It was hard to believe there was a ceiling there at all, and that the Great Hall didn't simply open on to the heavens.
--PS7
Some wizarding institutions have enchanted ceilings in their largest and most important rooms.
The ceiling of Hogwarts' Great Hall is enchanted so that it mimics the sky outside (PS7). Lightning blasts across it during thunderstorms (GF12). It has appeared deep black and full of stars (PS7, PA9) and covered with clouds of pewter gray (GF13). It has glowed red-gold at sunrise (DH36). At times the enchantment has been modified for a special occasion. Warm, dry enchanted snow fell from it for a Christmas celebration (CS12) and colourful confetti rained down during Lockhart's Valentine's Day festivities (CS13). The ceiling of the Atrium of the Ministry of Magic also seems to be enchanted to resemble the sky outside. golden symbols continually move across it "like some enormous heavenly noticeboard" (OP7); peacock blue early in the morning (OP7); dark blue in the evening (OP34)