if 6-7 months passed in the manga since yona's birthday (april), does that mean hak's birthday (august) already passed ? and they didn't even acknowledge it !! :(
Though the characters’ birthdays have been given to us by sensei, and some birthdays are mentioned in the series (see Kija’s and Soo-Won’s) I don’t think birthdays really play a big role in the story of AkaYona. What I mean by this is that they definitely could be addressed in canon, but unless they are consequential to the plot, there is no real reason to include them or stop the main narrative to address them, and I understand why Kusanagi-sensei wouldn’t bother to do so.
As such, I doubt everyone ignored Hak’s (and everyone else’s) birthdays, but rather that we just never see them celebrate. Especially as the main story of AkaYona typically follows serious events, it would be a bit of a shock theme-wise for everyone to suddenly stop for a chapter to have a party.
At least this is how I see it :)