In both Legends and current canon, Luke and Leia had been told at least some true information about their origins while growing up:
- Luke knew that Owen and Beru were his aunt and uncle, and that his father's name was Anakin Skywalker, although he was told that his father had died and was a "navigator on a spice freighter".
- Leia knew that Bail and Breha were her adoptive parents, as evidenced by Luke asking if she remembers her "real mother" in Return of the Jedi.
However, they don't seem to have had any suspicion that they might be related until Return of the Jedi, even though they were born on the same day of the same year and both knew they were both adopted under mysterious circumstances. Even if the Rebel military didn't celebrate birthdays, Leia would presumably have access to everyone's personnel file as a Rebel officer. This raises the question of whether they knew the real day (or location) of their birth, or whether their adoptive guardians told them something else.
Did Luke or Leia know when (or where) they were born? Answers from canon and Legends are both acceptable.