Tomorrow is Shizuru's Birthday and it's Saturday.
Birthday = Saturday (I'm so good at math, aren't I?)
Satur is pronounced as shaturu in Japanese and so it becomes shatter.
To Shatter means to suddenly break something into small pieces.
Breaking can also mean Braking (reverse of Kotarou's nice hearing).
We find brakes in cars.
Nishikujou Touka's car is red.
Blood is dark red.
Wars are bloody.
Wars cause humans to die.
Humanity dying could be seen as the End of the World
And so, the result is:
Tomorrow is the end of the world since it's Shizuru's birthday.
To celebrate the cute loli saury lover Nakatsu Shizuru's birthday. many great celebrities from many great lands somehow managed to cooperate and hold a birthday party. Saury was an important ingredient in the cake. The pop music that Shizuru loved so much was the BGM. A huge logo could be seen in the entrance that had Shizuru's famous "Dekiruyo" pose on it accompanied with the sentence "Shizuru's Great Birthday Party, we can do it!"... But of course, they didn't just prepare all of this for celebrating her birthday only. Many were thinking that they'd have the chance to get to know the Great Loli Goddess through holding it and preparing her the greatest present ever. They even decided on their own that whoever's present makes Shizuru the happiest would have the right to propose to her. But, as you already know, Shizuru doesn't really say whether she's happy about a particular gift... I mean, not everybody's like Kotarou and can understand her through her expressions, you know? They couldn't really decide whoes present made Shizuru the happiest and so they fought... and fought... and fought a lot. At first it was, a fight between a group of people, and then between countries and then it turned into a worldwide war. And so showed up Kagari and killed the survivors.
Justas a side note, Kotarou didn't show up in this story because it happens after Shizuru's route and he already turned into a tree back then.