Super Mario Galaxy 2 took place all in one day because of the wormholes the ship went through to get to each world, as opposed to actually flying to each galaxy. Also, the ending took place at night, while the intro took place in the day, so my theory could be possible. In fact, when Mario actually got back to the Mushroom Kingdom, it was at sunset. Then again, I'm talking about a game series where you can kill the sun with a turtle shell. I'm probably over thinking it.