"At evening time, there will be light"
Rabbi Yosi describes how there will be a day without day or night, until the evening, when there will be light, at which time, two evils will befall Jacob's children. But these occur so his children can be taken to "the good land." Rabbi Aba then poses a conundrum: "What is darkness?" The others determine that "darkness is Gehenom."
The phrase, "day without day or night," refers to the potential darkness and destruction [God forbid] that may appear before the final redemption of mankind takes place. Two paths to redemption and eternal peace are always available to us, a Light - filled path that embodies mercy and protection, or a darkened path that embodies pain, torment, and suffering. Our egos constantly urge us towards the path of darkness. Through a meditative, intense reading of this passage, we can help ourselves, and the world at large, avoid the potential gloom and devastation that attends the path of darkness.