User blog comment:Kaboomblahstick/Question: Hazel's birthday/@comment-3146930-20120103113127

Except your missing a key point. Even if she was born in December, she died and time effectively stopped for her. When Nico found her and brought her back to the land of the living, unless he brought her back on the same day she died she would have a different date when she became a year older. Much like Thalia, who was a tree and as brought back with a different B-day (as she wasnt sure when her birthday was anymore).

Hazel may be 14 years old, but she may not know when her Birthday is herself and is simply skipping a birthday so that she can start fresh when it circles back around to December.