Most teenagers love a sunny day they can spend with friends.
But when you're a mutant turtle the sun doesn't mean much to you, you never get to be out in it anyway.
Raph loved to laze about and sleep the day away. He only ever looked forwards to the sunsets.
During the day the world belonged to the humans that lived there. But as soon as the sun kisses the horizon, the city becomes their playground. Shadows to hide in and no reason to hide in the sewers.
After sunset was when they got to be free without fear.