But isn't kaiba adopted? how does he know his birthday????
either he has access to his Egyptian self's birthday, or he made one up to compensate for his lack of real parents
a possible reason is that his parents sent him to the orphanage and the people there kept track of his personal data, and when he was older they told him his birthday, why on earth would you assume that just cause he was adopted he wouldn't know his birthday??
Actually, Seto and Mokuba had parents, but they died not long after Mokuba was born. After their death both brothers ended up in the oprhanage. It's in the manga.
He was sent to the orphanage by his relatives after his father died in an accident and they used up all his and Mokuba's inheritance. He was 8 back then. This story was more clear in the original Japanese dub and in the manga!