Due to being immortal and isolating himself for a long, long time, Shiro hasn't really bothered to celebrate his birthday. Kuroh, Neko, and Hieda decided that they should do so this time. Shiro's other students wanted in on it too cause they care about their teacher very much.

Aw, this would be sweet. I could definitely see Shiro not bothering to celebrate his birthday, like he spent so many years alone in his airship that he just stopped counting birthdays or even thinking about them all together. One day one of his students asks how old he is anyway and Shiro makes a joke about never asking adults their age. Later when he’s back at the apartment Shiro mentions this exchange to Kuroh and Neko and admits that he’s not actually sure how old he is, trying to math it out like wait my birthday is June 1st on…and he can’t even entirely remember the year. Kuroh wonders how Shiro could forget that and Shiro just shrugs, he never celebrated birthdays after becoming a King so it’s easy to forget. 

Later after Shiro’s gone to class Kuroh finds Neko digging around in the closets, looking for supplies left over from the school festival. Kuroh asks what she’s doing and Neko says she needs things for a party because isn’t it almost June 1st. Kuroh is confused for a moment and then he remembers that conversation with Shiro and asks if Neko wants to give Shiro a party, Neko nods and says it’s fun to celebrate birthdays and so they should celebrate with Shiro. Kuroh considers this and says that’s an excellent idea, imagine him immediately going into party planner mode like he’s got a notebook and he’s writing down that we need streamers and ingredients for a cake and presents while Neko stews because planning is boring, let’s just throw glitter everywhere and have a fun party. Kuroh is trying to impress upon her the importance of proper planning when Hieda stops by and asks what’s going on.

Kuroh explains that they want to throw Shiro a party and of course this quickly gets out to the rest of the students, imagine Kukuri joining Kuroh on the unofficial party planning committee and now they’re suddenly having secret meetings in a lecture hall with a bunch of the other students plotting out a giant surprise party. Shiro is vaguely aware that something is definitely up, because Kuroh and Neko probably can’t keep secrets to save their lives (Kuroh is too honest and Neko has a tendency to just blurt things out when she’s excited) but he figures he’ll find out soon enough. I imagine Shiro keeps assuming that maybe some of the students roped the two of them into something or it’s going to be some sort of prank, the idea that they’re planning to throw him a birthday party never crosses his mind.

Finally it’s Shiro’s birthday and he heads to his first class, when he steps into the classroom suddenly there’s confetti thrown in his face and Neko yelling happy birthday. Kuroh scolds her that this was supposed to be a surprise, we were all supposed to jump out at the same time and Neko says Kuroh was too slow. She clings to Shiro as she’s like come on look at the decorations and we made a cake and there are crackers, very excited to show him his birthday party. Shiro is just totally caught off guard as all his students come over to wish him a happy birthday, sitting down in front of his cake as Kuroh properly ties a party hat onto his head. Kuroh admits that they didn’t know how many candles to put on so he just put one, to symbolize Shiro’s new life. Neko asks if Shiro liked the party and was he surprised, Shiro laughs finally and says he was definitely surprised. He looks around at everyone, his students and his clansmen, all the people who care about him and Shiro isn’t even sure if he wants to blow out the candle since it seems his wishes have already come true.