Curate, connect, and discover
Given his past, I imagine Chuuya's relationship with his birthday must be a bit unusual. To him, his birthday is a sign of his own humanity, a sign that he is a human born like everyone else, and not just a clone created in a lab. He also only found out about his birthday because of the flags, so this is also a symbol of his friendship with them, and how much they cared for him, being able to go against Mori's orders just to give Chuuya the knowledge about his own origin, the thing he desired the most