The title is a nod to this post-makeout banter between Natasha and Daniel (321):
"You poets are obsessed with the stars. Falling stars. Shooting stars. Dying stars."
"Stars are important," I say, laughing.
"Sure, but why not more poems about the sun? The sun is also a star, and it's our most important one. That alone should be worth a poem or two."
"Done. I will only write poems about the sun from now on," I declare.
So why does this one random quote snag top billing in the book? Yoon never directly explains it, but we'll grab our lucky philosopher's cap (it's bedazzled) and take a crack at the deeper meaning.
There are 200 billion stars in the Milky Way, but only one is bright enough and strong enough to be the sun. Similarly, Natasha and Daniel's lives are made up of thousands of days, but this particular day is significant enough and special enough to change their orbits entirely.