No other monarch in British history has achieved 70 years of service.
The Queen, then aged 25, acceded to the throne on February 6, 1952 on the death of her father, King George VI.
She became the longest-reigning British monarch in 2015, beating the time spent on the throne by her great-great-grandmother Queen Victoria, who ruled for 63 years, seven months.
In recognition of the landmark occasion, Buckingham Palace is pulling out all the stops. Here's what you need to know.
To celebrate the unprecedented anniversary, a number of events have taken place around the UK this year. It all culminates in a four-day national bank holiday weekend from Thursday, June 2 until Sunday, June 5, known as the Platinum Jubilee Weekend. In the UK, royal celebrations are typically held in the summer to allow for better weather.
The weekend will feature a variety of public events and community activities, as well as "national moments of reflection" on the Queen's seven decades as sovereign, according to the palace.
The upcoming celebrations will be the Queen's first jubilee without her husband, Prince Philip, who died in 2021.
Several jubilees have punctuated the Queen's reign since her accession to the throne in 1952: the Silver Jubilee, marking 25 years in 1977; the Golden Jubilee celebrations for 50 years in 2002; and the Diamond Jubilee commemorations a decade ago for her 60th anniversary.
The monarch opted to mark other anniversaries, like her Ruby Jubilee (40 years in 1992) and Sapphire Jubilee (65 years in 2017), with less fanfare and without public events.
The Queen's private estates -- including Sandringham House and Balmoral Castle -- are also joining in with jubilee themed events.
Thursday, June 2
The festivities kick off from 10 a.m. BST (5 a.m. ET) with the Queen's birthday parade, known as Trooping the Colour. The annual ceremony is returning to central London after a two-year hiatus due to the coronavirus pandemic.
In an impressive display of military pageantry, more than 1,200 officers from the Queen's personal troops, the Household Division, will be joined by several hundred Army musicians and 240 horses. The "colour" -- or regimental flag -- will be trooped by the 1st Battalion, Irish Guards. The procession will start at Buckingham Palace and move down The Mall to Horse Guard's Parade, joined by members of the royal family on horseback and in carriages.
Upon returning from the parade ground, the Queen and members of the royal family will make their customary balcony appearance. The event will close with a fly-past over the palace.
Later, 1,500 beacons will be set alight across the UK, the Channel Islands, the Isle of Man and in UK Overseas Territories. The principal beacon will be lit in a special ceremony at Buckingham Palace. The lighting of beacons is a long running royal tradition used to mark jubilees, weddings and coronations. Beacons will also be lit in the capital cities of Commonwealth countries.
Friday, June 3
A thanksgiving service paying tribute to the Queen's lengthy reign will be held at St Paul's Cathedral with family members in attendance.
Saturday, June 4
Several royal family members are expected to head to Epsom Downs racecourse in the afternoon for the 243rd edition of its famous horse race, the Derby. The Queen, a keen horse breeder herself, has been a regular spectator at the event and has even presented the famous trophy in years gone by.
In the evening, a two-and-a-half hour "Platinum Party At The Palace" concert will see a star-studded line up take to three stages built in front of Buckingham Palace and the famous Queen Victoria Memorial. Queen + Adam Lambert, Alicia Keys and Diana Ross are among the artists set to perform their biggest hits at the show, which will be broadcast live by the BBC. Some 22,000 people will get to watch the concert in person, including 10,000 who won tickets through a public ballot; 5,000 tickets were reserved for key workers.
To cap the celebrations, on Sunday, people are being encouraged to organize street parties as part of the "Big Jubilee Lunch" initiative. Community gatherings are set to take place across Britain, including flagship events in London and at Cornwall's Eden Project -- where the idea for the lunches originated. "Big Jubilee Lunches" have also been planned around the world, from Canada to Brazil to South Africa and Japan.
The weekend's finale is the Platinum Jubilee Pageant, in which artistic performers, dancers, musicians, military personnel, key workers and volunteers will unite to bring iconic moments from the Queen's reign to life in a festival of creativity. Starting at 2:30 p.m. (9:30 a.m. ET), the pageant will involve a "River of Hope" section that will comprise 200 silk flags parading down The Mall like a river. School children have been invited to create a picture of their hopes and aspirations for the planet over the next 70 years, a selection of which will be shown on the silk flags.
It's still not exactly clear when we'll see the Queen over the weekend.
The 96-year-old monarch has struggled with mobility issues recently and has been forced to withdraw from several public appearances, including the State Opening of Parliament in May.
She'll continue to play it safe for the jubilee and hasn't confirmed whether she'll be present at the various festivities.
A royal source recently told CNN that Her Majesty is "looking forward" to the revelry and plans to take part in the celebrations, but "her presence will not be confirmed until much nearer the time or on the day itself."
Most senior royals are expected to attend a few of the jubilee weekend events in central London. Some will also deploy to all four UK nations during the four-day extravaganza, with the Queen sending the Cambridges to Wales, the Earl and Countess of Wessex to Northern Ireland and Princess Anne to Scotland.
And after much speculation, it has also been confirmed that the Duke and Duchess of Sussex, and their children, will be flying back to the UK for the celebrations.
The Queen has decided that only royals carrying out official duties will make an appearance on the famous Buckingham Palace balcony during the Trooping the Colour parade on Thursday. So we are expecting to see the Queen alongside three of her children -- Charles, Edward and Anne -- as well as Prince William and Kate and their children, and a number of the monarch's other relatives.
This doesn't mean Harry, Meghan or Andrew won't be involved in the celebrations at all. The wider family is traditionally invited to church services, like the one set for St. Paul's Cathedral on the Friday.