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What we know about Queen Elizabeth II's health after she pulls out of jubilee thanksgiving service

London(CNN) With Queen Elizabeth II missing a service of thanksgiving on Friday at St Paul's Cathedral for her 70-year reign, questions are swirling about the health of the United Kingdom's 96-year-old monarch.

Buckingham Palace said Thursday the Queen experienced "some discomfort" during the first day of her Platinum Jubilee celebration and would sit out of the second day's major event.

"The Queen greatly enjoyed today's Birthday Parade and Flypast but did experience some discomfort," Buckingham Palace said in a statement Thursday.

"Taking into account the journey and activity required to participate in tomorrow's National Service of Thanksgiving at St Paul's Cathedral, Her Majesty with great reluctance has concluded that she will not attend," it continued.

In years past, the monarch rarely scrapped engagements at the last minute. But her advanced age has made it difficult to work the jam-packed schedule, filled with official engagements and appointments, that has defined the majority of her reign.

As she has gotten older, the Queen has reduced her number of commitments to focus on core constitutional duties as the head of the Church of England and the Head of State, such as the opening of parliaments and audiences with the Prime Minister and new ambassadors to the UK. She began using a walking stick in public last year.

The palace has given no reason to think the Queen is in poor health. However, her age and the fact that she is the world's most recognizable woman often fuel speculation when something is amiss with the British sovereign. The palace has tried to strike a balance between being truthful while preventing people from becoming overly concerned.

Last year, officials moved quickly to ease worries over the monarch's health when she canceled a trip to Northern Ireland. Fears were immediately put to rest when a source close to the palace told CNN she was in "good spirits," but it then emerged that she had spent the night in hospital for what a palace spokesperson called "preliminary investigations."

Thursday's announcement was not completely unexpected. Even before jubilee events began, a royal source told CNN the monarch's presence would "not be confirmed until much nearer the time or on the day itself." And while the Queen's absence was surely felt in the cathedral Friday, she was watching the proceedings on television from Windsor Castle, a Buckingham Palace spokesperson told CNN.

"The Queen will be disappointed to miss today because her faith is so important to her. It always has been," said CNN's royal historian Kate Williams.

"Her role as Head of the Church of England. It's not always what we think about when we think about the Queen .... but it means so much to her. She swore to God on the day of her coronation ... so it'll mean a lot to her that there is this service and of course, she only couldn't come here if that's really absolutely the case."

Four days of celebrations to mark her seven decades on the throne began Thursday, with the Trooping the Colour -- a centuries-old traditional military spectacle involving 1,500 soldiers and officers, 400 musicians and 250 horses.

The Queen did not fill the monarch's ceremonial role of riding on horseback -- her son and heir, Prince Charles, filled in -- but she nonetheless delighted fans with her presence, looking on from the palace's famous balcony at a sea of red, white and blue as huge, flag-waving crowds flocked to the Mall to see her and her family. In the evening she participated in a beacon lighting ceremony from Windsor Castle.

Festivities will also include Friday's service, a concert at the palace on Saturday and a pageant on Sunday.