Priestess and author, Isidora Forrest has a wonderful post detailing the reason why Heru-sa-Aset’s birthday is celebrated on December 25 (Hint: Look in Plutarch). Here is the article: Isis, Osiris, Horus and the Holy Day of December 25.
The Kemetic Festival dates for the birth of Heru-sa-Aset (Horus son of Isis) are different.
The celebration of Giving Birth of Aset, Mother of God is from 4 Peret days 4-21 which is in March or April depending on the Kemetic calendar (1). And the Kemetic date of the festival titled the Birthday of Heru-sa-Aset (Horus son of Isis) is not on the Winter Solstice (I can’t remember the exact date off the top of my head). And there is also a Greek/Roman festival of Aset Births Heru-sa-Aset/Isis Births Horus the Younger on 4 Peret Day 28.
(1) El-Sabban, Sherif. Temple Festival Calendars of Ancient Egypt. (Wiltshire: Liverpool University Press, 2000), 176.