Princess Leonor of Spain is turning 18 this week. Queen Letizia and King Felipe’s firstborn will celebrate her milestone birthday on Tuesday, Oct. 31.

Like her father did on his 18th birthday in 1986, the heir to the Spanish throne will swear the Spanish Constitution on her big day. The ceremony will be attended by the Princess of Asturias’ parents and younger sister, Infanta Sofia, who is currently on break from UWC Atlantic until Nov. 5.

After swearing the constitution before the Cortes Generales, Princess Leonor will receive the Collar de la Real y Distinguida Orden Española de Carlos III (The Royal and Distinguished Spanish Order of Charles III) at the Royal Palace of Madrid.

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While a lunch will be offered at the palace, HOLA! Spain reports that King Felipe and Queen Letizia will host a private celebration for family at El Pardo palace later in the day.

Leonor, who is in the midst of her first year of military training, spoke about her upcoming birthday earlier this month at the 2023 Princess of Asturias Awards. At the ceremony, the future Queen of Spain said (translated to English): “On the 31st I will turn 18 and I will have the honor of swearing in the Constitution, with what that means for me personally and institutionally. What I can tell you is that I very well understand and am aware of what my duty is and what my responsibilities entail.”

Another future European monarch, Prince Christian of Denmark, recently celebrated his 18th birthday. On Oct. 15, the Danish Prince’s grandmother Queen Margrethe II hosted a gala dinner at Christiansborg Palace. Future Queens Princess Ingrid Alexandra of Norway, Princess Catharina-Amalia of the Netherlands, Princess Elisabeth of Belgium and Princess Estelle of Sweden were among the royals at the celebration in Copenhagen.