Seventy years ago this weekend, a young English princess climbed into Treetops, a remote game-viewing lodge in Kenya, built into the limbs of a fig tree overlooking an elephant watering hole. The next morning, she came down as a queen, though she only learned of the death of her father, George VI, later that day.

The 70th anniversary of Queen Elizabeth II’s accession to the throne will be a good deal more earthbound: The 95-year-old monarch plans to spend a quiet Sunday at her country estate, Sandringham, where her father died on Feb. 6, 1952. Four days of festivities to celebrate her Platinum Jubilee are scheduled for June.

But tributes to the queen poured in from Britain’s great, good and merely prominent. Justin Welby, the archbishop of Canterbury, told the BBC, “She takes her duties seriously, but she doesn’t take herself very seriously.” Prime Minister Boris Johnson praised her for her “inspirational sense of duty and unwavering dedication to this nation.”

Those words, from a scandal-scarred leader whose tenure in office might be measured in days rather than decades, were testimony not only to the queen’s longevity but also to her immutability. In a country that lurched from the storms of Brexit into the siege of the pandemic, she has been an unmatched anchor of stability.

Time, of course, has not spared Elizabeth, either. She is commemorating this milestone alone, having lost her husband of 73 years, Prince Philip, in April. And her health has declined in recent months, forcing her to cancel multiple public appearances, including a remembrance service for the war dead in November.

That was a painful blow to the queen, who served in the auxiliary service as a driver and truck mechanic during World War II. For worried Britons, it was another sign of her fragility and a wistful reminder that the second Elizabethan Age is coming to an end.

In a message issued on Saturday, the queen spoke candidly about a royal family in transition. And she delivered a surprise, in the form of an endorsement of her daughter-in-law, Camilla, the second wife of her son and heir, Prince Charles.

“When, in the fullness of time, my son Charles becomes King, I know you will give him and his wife Camilla the same support that you have given me,” the queen wrote. “It is my sincere wish that, when that time comes, Camilla will be known as Queen Consort as she continues her own loyal service.”

That settled a longstanding and delicate question about whether Camilla, who was romantically involved with Charles during his marriage to Princess Diana, would ever have the title of queen. It is a victory for Charles, who long pushed for that recognition for his wife, now known as the Duchess of Cornwall.

Elizabeth has otherwise endured another bumpy stretch in the soap opera that is her family. She recently stripped her second son, Prince Andrew, of his honorary military titles, as he fights a lawsuit in a New York court on accusations that he sexually abused a teenage girl while a guest of the disgraced financier Jeffrey Epstein.

Her grandson Prince Harry and his wife, Meghan, remain estranged from the family, with Harry at work on a memoir that palace officials worry will reopen the wounds from a bitter interview that the couple gave to Oprah Winfrey last year. The queen has yet to meet her great-granddaughter Lilibet, named in honor of Elizabeth, whose parents called her by that nickname.

The queen, however, remains enduringly popular: Her 76 percent approval rating is No. 1 among the royals, according to a poll last year by the market research firm YouGov. Charles polled at 45 percent; Prince William, the next in line, at 66 percent; and the once-popular Harry at 39 percent.

“She has an instinctive understanding of the soul of the British people,” said Vernon Bogdanor, a professor of government at King’s College London. For all of the upheaval in the House of Windsor, he added, “The monarchy is seen as a unifying force of stability and of constitutional democracy.”

Reaching this milestone puts Elizabeth in rare company. Only three monarchs are documented to have reigned more than 70 years: Louis XIV of France; Johann II of Liechtenstein; and Bhumibol Adulyadej of Thailand, who died in 2016. Elizabeth is already the longest-serving British monarch, overtaking Queen Victoria in 2015, and the longest-serving female monarch. She would surpass Louis XIV, the Sun King, in less than three years.

Her reign spans the post-World War II era. When the queen welcomed President Biden to Windsor Castle in June, he became the 13th American president to meet her. She has met every president since Harry S. Truman, save for Lyndon B. Johnson.

She has been served by 14 prime ministers, starting with Winston Churchill. If the political handicappers are to be believed, she may soon be on her 15th. An outcry over gatherings held in Downing Street that breached pandemic lockdown restrictions has led to calls for a no-confidence vote in Mr. Johnson.

Some Key Moments in Queen Elizabeth’s Reign

A historic visit. On May 18, 1965, Elizabeth arrived in Bonn on the first state visit by a British monarch to Germany in more than 50 years. The trip formally sealed the reconciliation between the two nations following the world wars.

First grandchild. In 1977, the queen stepped into the role of grandmother for the first time, after Princess Anne gave birth to a son, Peter. Elizabeth’s four children have given her a total of eight grandchildren, who have been followed by several great-grandchildren.

A trip to Ireland. In May 2011, the queen visited the Irish Republic, whose troubled relationship with the British monarchy spanned centuries. The trip, infused with powerful symbols of reconciliation, is considered one of the most politically freighted trips of Elizabeth’s reign.

Breaking a record. As of 5:30 p.m. British time on Sept. 9, 2015, Elizabeth II became Britain’s longest-reigning monarch, surpassing Queen Victoria, her great-great-grandmother. Elizabeth was 89 at the time, and had ruled for 23,226 days, 16 hours and about 30 minutes.

Marking 70 years of marriage. On Nov. 20, 2017, the queen and Prince Philip celebrated their 70th anniversary, becoming the longest-married couple in royal history. The two wed in 1947, as the country and the world was still reeling from the atrocities of World War II.

Perhaps his lowest moment was having to apologize to Buckingham Palace for two raucous parties thrown by his staff on the night before Philip’s funeral. The next day, a photographer captured an image of the queen, grieving alone at the service, masked and isolated in a choir stall at St. George’s Chapel.

If Mr. Johnson hangs on until June, when the Platinum Jubilee is celebrated, he might even benefit from the general atmosphere of celebration. Among the events planned is a carnival-like procession of 5,000 performers through the streets of London, led by a dragon puppet the size of a double-decker bus. The government will give everyone an extra day off.

Accession Day, however, has always been a melancholy anniversary for the queen, as much about the death of her father as her own ascension to the throne. Although George VI had been seriously ill, his death was traumatic for the 25-year-old princess, who was by all accounts very close to him.

Still, Elizabeth managed some low-key festivities on Saturday, cutting a cake and playing host to members of volunteer groups. Among her guests was Angela Wood, an 88-year-old onetime cooking student, who created “coronation chicken,” the dish served to 350 V.I.P.’s at the banquet on coronation day in 1953.

Mrs. Wood and the queen discussed the recipe, which calls for diced chicken, tomato paste, a dash of curry powder, brown sugar, a pinch of salt, a splash of red wine, later mixed with mayonnaise and puréed apricots.

“For a month or more,” she told the BBC, “I was cooking a chicken a day, and we had to alter the balance of the spices in the sauce to get it right.”