The Union flag is flying at Belfast City Hall to mark the Duchess of Cambridge’s 31st birthday.
Paul Faith, PA Wire
It is the first time the flag has been raised since a decision not to display it permanently sparked sometimes violent protests.
The Duchess of Cambridge will be spending the day privately, St James’s Palace has told MSN News.
It will be her last birthday before she and her husband, Prince William, welcome the arrival of their first child.
But how are the birthdays of other members of the royal family celebrated?
The Queen’s two birthdays
The Queen celebrates her actual and official birthday every year.
Her birthday is on 21 April and she usually spends it privately. But the day is marked publicly at midday with a 41-gun salute in London’s Hyde Park, a 21-gun salute in Windsor Great Park and a 62-gun salute at the Tower of London.
Her official birthday is on a Saturday in June. Other British monarchs have also enjoyed official birthday celebrations other than on their actual birthday, particular when the latter has not been in the summer.
The idea is that there is a greater likelihood of good weather for the birthday parade, also known as Trooping the Colour.
The Queen is joined by other members of the royal family at the parade, which moves between Buckingham Palace, the Mall and Horseguards’ Parade in London.
She celebrated her 80th Birthday in 2006 with a walkabout outside of Windsor Castle to meet well-wishers.
Tens of thousands of civil servants also get a day off to mark the Queen’s birthday.
Prince Charles and his 64th “Hobbit” birthday
The Prince of Wales celebrated his last birthday in New Zealand in November 2012 with an unusual present.
During a sneak preview of costumes and props from The Hobbit film, a dwarf from the movie offered his sword and service to the royal.
Laughing, Charles said: "This is the best birthday present I've had for a long time."
During the day, he cut his birthday cake to the strains of the Beatles tune, When I’m Sixty-Four. And he was joined by 64 New Zealanders of all ages who were also born on November 14.
New Zealand was the final stop for Charles and the Duchess of Cornwall on their Pacific tour to mark the Queen's diamond jubilee.
The celebrations for Prince Philip
As with other members of the royal family, the Union flag is flown from government buildings on the Duke of Edinburgh’s birthday.
A salute of 41 guns is also fired on his birthday in London’s Hyde Park.
He’s also been honoured with a 62-gun salute at the Tower of London, including on his 90th birthday in 2011.
The Royal Mint produced a £5 commemorative coin in his honour that year. It featured for the first time an image of a consort on one side and a reigning monarch on the other.
The royal family also attended a special service of thanksgiving to mark the Duke of Edinburgh's 90th birthday. More than 750 guests were invited to the service at Windsor Castle's St George's Chapel. The Duke was born on 10 June 1921.
The national anthem is also played on BBC radio 4 at 7am to mark the duke’s birthday as well as those of the Queen and the Prince of Wales.
Duke of Cambridge
Prince William marked turning 30 last year by being pulled in a rubber ring behind a speedboat near his Welsh home, it was reported.
The Sun said that his wife Kate is said to have booked the high-speed rubber “doughnut” ride – with the Duchess of Cambridge, Prince Harry and a group of close friends roaring with laughter on the day itself.
The prince, who was born on June 21, 1982, also inherited £10 million from his mother Diana’s estate when he turned 30.