COPENHAGEN, Denmark (AP) — Thousands of people will gather in downtown Copenhagen on Sunday to witness a historic moment in one of the world’s oldest monarchies.

Around 2 p.m. (1300 GMT), Queen Margrethe II will sign her abdication and about an hour later her eldest son will be proclaimed King Frederik X on the balcony of Christiansborg Palace in the heart of the Danish capital.

Margrethe, 83, will become the first Danish monarch to voluntarily relinquish the throne in nearly 900 years.

She will formally sign her abdication at a state council — a meeting with the Danish Cabinet — which automatically makes the crown prince, 55-year-old Frederik, Denmark’s new monarch.

Citing health issues, Margrethe announced on New Year’s Eve that she would step down, stunning a nation that had expected her to live out her days on the throne, as is tradition in the Danish monarchy.

Margrethe underwent major back surgery last February and didn’t return to work until April.

Even Prime Minister Mette Frederiksen was unaware of the queen’s intentions until right before the announcement. Margrethe had informed Frederik and his younger brother Joachim just three days earlier, the Berlingske newspaper wrote, citing the royal palace.

“The queen has been here and she has been on the throne for more than 50 years. And she is an extremely respected and popular figure, so people hadn’t expected this and they were in shock,” Danish royal expert Thomas Larsen said.

The last time a Danish monarch voluntarily resigned was in 1146, when King Erik III Lam stepped down to enter a monastery. Margrethe is abdicating 52 years on the day after she ascended the throne following the death of her father, King Frederik IX.

Denmark’s monarchy traces its origins to 10th century Viking king Gorm the Old, making it the oldest in Europe and one of the oldest in the world. Today the royal family’s duties are largely ceremonial.

Unlike in the UK, there is no coronation ceremony in Denmark. The prime minister will formally proclaim Frederik king from the balcony of Christiansborg Palace, which houses government offices, Parliament and the Supreme Court as well as the Royal Stables and Royal Reception Rooms. Thousands of Danes are expected to witness the proclamation from the square below.

The abdication will leave Denmark with two queens. Margrethe will keep her title while Frederik’s Australian-born wife will become Queen Mary. Frederik and Mary’s eldest son Christian, 18, will become crown prince and heir to the throne.

The new king and queen will leave Christiansborg Palace in a horse-drawn coach and return to the royal residence, Amalienborg, where Margrethe also lives but in a separate building. The royal standard will be lowered on Margrethe’s home and raised on the building where Frederik and Mary live.

Four guns on the Copenhagen harbor will fire three times 27 rounds to mark the succession. In the late afternoon, Copenhagen’s Tivoli Gardens amusement park plans to celebrate the new king and queen with the biggest fireworks show in the park’s 180-year history.