TO BE FAIR. HE PROBABLY DIDN'T IMMEDIATELY REMEMBER THAT THIS IS THE MONTH THAT'S THE EQUIVALENT OF THEIR APRIL or whatever the month would actually be called there. Especially since it was nowhere close to her birthday when they were last together. I doubt even Yona would know that it was her birthday then.

I do remember last year Hak had given Yona something during the festival (although it was never played out haaa) so he'll probably remember it and be like "Well might as well" and give her birthday present then.