Friday is the second day of Queen Elizabeth's Platinum Jubilee celebrations in the United Kingdom, with the highlight being a thanksgiving ceremony attended by senior royals and lawmakers. However, the 96-year-old monarch will be absent owing to ongoing mobility concerns.

The four-day celebration commenced on Thursday when a smiling Elizabeth waved to the audience from the balcony of Buckingham Palace following a military parade and Royal Air Force flypast and later oversaw the lighting of the Principal Platinum Jubilee Beacon at her Windsor Castle residence.

A National Service of Thanksgiving will be held at London's St Paul's Cathedral to commemorate the monarch's 70 years on the throne.

The queen, who has recently been forced to postpone several engagements due to "episodic mobility problems," will be absent, withdrawing late on Thursday and diminishing the festive ambiance of the day.

"The queen greatly enjoyed today's Birthday Parade and Flypast but did experience some discomfort," according to a statement from Buckingham Palace.

The drive from Windsor Castle, where she spends most of her time, to London and the service-related activities were deemed taxing by officials, who made a regrettable but understandable choice.

According to a royal source, the queen's attendance was never a foregone conclusion but rather her hope.

She won't be the only one absent. Prince Andrew, her second son, has tested positive for COVID-19 and will miss the service, a spokeswoman for Buckingham Palace announced Thursday.

This may spare the royals some difficulty, as Andrew's reputation has been damaged since he settled a U.S. lawsuit in February. He was accused of sexually abusing an underage woman, charges he denies.

Prince Harry and his wife Meghan, who have made almost no public appearances in Britain since stepping down from royal duties two years ago, are anticipated to attend.

The pair relocated to the United States to pursue a more independent life, and they have since launched a series of vicious attacks against Buckingham Palace and the royal family.

Bible readings, prayers, and hymns will be included in the ceremony to express thanks for Elizabeth's rule. British and international political figures will be present, and Prime Minister Boris Johnson will deliver a reading.

David Ison, the Dean of St. Paul's, will say, "We come together in this cathedral church today to offer to God our thanks and praise for the reign of her majesty the queen and especially for her seventy years of faithful and dedicated service,"

The cathedral's 'Great Paul' bell, which dates back to 1882 and is the largest in the country, will also be rung for the first time during a royal event since it was refurbished last year following a mechanical failure in the 1970s.

After the service, the Lord Mayor of the City of London will hold a reception at the Guildhall.

Thursday marked the beginning of the Jubilee and the 69th anniversary of Elizabeth's coronation, which occurred in February 1952, following the death of her father, George VI.

She has remained on the throne longer than any of her predecessors in the past thousand years and is the third-longest reigning monarch of a sovereign state. Opinion polls indicate that she continues to have immense support and esteem among British citizens.

"She is an extraordinary person in our lives and always has been," said Sandra Wallace, a 74-year-old retired teacher among the tens of thousands who flocked to central London on Thursday.