wonderful idea i just had: joll and conk are canonically the same age, but i dont think we ever had a confirmation on their canon birthdays??? (someone tell me if they do but anyway)
i imagine joll doesnt know his actual day of birth since A. he was stolen from water civ as a baby and B. literally No One thought it was important to tell him + the only time he interacted with his bio parents was like. overshadowed by the fact that they were in Danger Again 😭
for conk it's either A. he forgot, or B. donk didnt see any point in celebrating it. either that or it was both. there's a big chance it was both.
anyway, i like to think after they met, they both did some rough calculations on their ages (mostly conk because joll calculated his own age when he was younger) and after a LONGG time of trying to make conk remember and figuring out timelines, they find out that surprise surprise! they're the same age!
so afterwards, since they need an actual day to celebrate and remember how old they were now, one of them suggested they chose the same day so it'll be easier. or because they wanted to be "twins". either option could work!
also also: they mentioned it Once to the gang and suddenly joll and conk gets presents from the team on their birthday when they barely remember mentioning it (golem grumbles about having to get two gifts but gives it to them either way/he gives them one gift so they could share)
joll and conk also like to give each other gifts on their birthday :)) conk usually gets steak or tnt, and joll usually gets a new sword or flint and steel
and because i get to be self indulgent: conk forgets a lot of things but he never forgets their birthday because its something he has in common with joll <3