Every Night & every Morn Some to Misery are Born. Every Morn & every Night Some are Born to sweet Delight. Some are Born to sweet Delight, Some are born to Endless Night. (119-124)
The symbolism here is pretty straightforward—"Endless Night" is suffering and misery. But you can be born to "endless night" when you get up in the morning or when it's late in the evening. It doesn't matter. It's a state of mind, not the literal time of day. "Sweet Delight" is the opposite—it's a spiritual day-time, full of joy.
Blake is saying that our lives are defined by these shifting states. We go from being sad to happy and back again—the process doesn't end… unless you find some way to break out of it. For Blake, the way to get out is a mystical and artistic practice—you can visualize the spiritual world through artwork or actually see it through mystical experiences.