During Divination class with Professor Trelawney, she takes a guess towards Harry's birth month.
"I think I am right in saying, my dear, that you were born in midwinter?"
Harry replies, "No, I was born in July."
Now that may seem like another one of Trelawney's failed predictions, but in the last book, we learn that Harry was one of Voldemort horcruxes. When was Voldemort born? December 31st, or Midwinter. So it is entirely possible that Trelawney sensed Voldemort's presence in Harry, and that's why she mixed up the dates.
— Quora user Yedu Krishnan