1)
What was Pharaoh's "Yom Huledes"?
1.
Rashi and Targum Onkelos: It means Pharaoh's birthday. 1
2.
Radak #1, Rabeinu Bachye: The day a son (a future Pharaoh) was born unto him.
1
Torah Temimah (note 9): That is the way the Yerushalmi (Avodah Zarah 1:2) interprets "Yom Ginusya" (see Targum Yonasan and Perush Yonasan); whereas the Bavli interprets it as the anniversary of his coronation day.
2)
What are the ramifications of the fact that Pharaoh celebrated his birthday?