In the story, Harry Potter’s 11th birthday falls on a Tuesday. The date is July 31, and the weekday is stated during the opening events surrounding the arrival of his Hogwarts letters and Hagrid’s visit.
Harry’s birthday is significant because it marks the turning point from his miserable life with the Dursleys to his discovery of the wizarding world. On or around his birthday, Hagrid tells Harry that he is a wizard, delivers his Hogwarts acceptance letter, and takes him to Diagon Alley for school supplies.
A common mix-up comes from checking the real-world calendar: July 31, 1991, falls on a Wednesday, not a Tuesday. The story’s internal dialogue nevertheless identifies Harry’s 11th birthday as Tuesday, so Tuesday is the canonical quiz answer. The film adapts this birthday sequence from the novel, whose wording supplies the weekday detail.