After her 90th birthday previous year, which saw national festivities held in her honour, the 2017 milestone will seem more subdued.
And famously, on her 21st birthday, the Queen gave a speech from South Africa where she pledged her "whole life whether it be long or short shall be devoted to your service and the service of our great imperial family to which we all belong".
On February 6 2017, she became the first British Sovereign to reach their Sapphire Jubilee, a reign lasting 65 years.
Prince Charles, her eldest son, is the heir to the throne, but his two sons, William and Harry, have been garnering bigger attention, and taken some of the Queen's duties.
In Hyde Park at midday, 41 volleys in honour of her birthday will be fired by the King's Troop Royal Horse Artillery.More news: Trump to hold rally on night of White House Correspondents Dinner
A further 62 Volleys were also fired at the Tower of London.
It will be marked with a parade in London known as the Trooping the Colour ceremony - this year's takes place the week later, on 17 June.
Her Majesty Queen Elizabeth II turns a staggering 91 today and is more relevant and necessary than ever.
The Queen owns two Corgis, who are called Willow and Holly.
The Queen has seen 13 different prime ministers pass through 10 Downing Street - from Winston Churchill to Theresa May, according to the Associated Press.More news: Today Is Prince's One Year Anniversary
Queen Elizabeth II's birthdays have commanded some giant celebrations in the past.
21 - age when she married the Duke of Edinburgh in November 1947. While we shed the monarchy and nobility when we gained independence, it does not seem to have dampened enthusiasm or interest in the Queen and her family.
The monarch has given herself the day off from formal engagements and will mark the milestone privately at her home near London.
Lee Cohen, New York Director of The Anglosphere Society, spent years advising the U.S. House of Representatives Foreign Affairs Committee on Western European affairs, and was founding Executive Director of the House United Kingdom Caucus.
The Queen didn't plan anything public for today and she will probably honor this special occasion with her nephews and grandnephews, out of sight.More news: Tesla to recall 53000 cars over parking brake issue