The Queen - whose full official title is Elizabeth the Second, by the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith - is spending the day in Scotland on relatively low-key appointments away from the limelight.
That didn't stop companies from getting involved, with many on Twitter using the hashtag #longestreign.
Kellogg's got in on the act early, getting press coverage last week with the creation of special 'Queen Flakes' - not dissimilar to existing offering Corn Flakes. Dubbed the "world's poshest cereal" by Kellogg's, Queen Flakes go on sale at Manchester’s Black Milk Cereal Café today.
— Kellogg’s UK & IRE (@KelloggsUK) September 5, 2015
Meanwhile, sugar refiner Billingtons has been baking, and offered a recipe of what it claims is the Queen's favourite biscuit.
Tesco used the day to reignite the debate about the order in which clotted cream and jam should be put onto a scone.
The digital analytics and mapping firm Esri UK has created an interactive map of every country Her Majesty has visited, including information on each destination.
Carlsberg played with its well-known 'probably the best...' slogan.
— Carlsberg UK (@CarlsbergUK) September 9, 2015
The English rugby team, currently preparing for the World Cup, showed their patriotism.
The video and animation agency Vox Pops International has created this video of five "royally interesting" facts about the Queen.
The online magazine Bloomberg Business looked into the royal finances.
Tourism body Visit London has been promoting various attractions and one-off events with royal links.
The National Theatre looked back to what it thought would have been a highlight of the Queen's six and a bit decades on the throne.
...and the Jewish charity Norwood also remembered one of several royal visits it has received.
Social media analytics service Brandwatch React found "overwhelming love" for the Queen today on social media.
Finally, a warning to Twitter users not to be fooled by the popular spoof account @Queen_UK, which has been using the day and the hashtag to promote a range of merchandise it sells. The gin loving parody Queen has 1.29 million followers on Twitter, beating the official @BritishMonarchy account by a margin of 30,000.
To do this week: 1) open a train line 2) bollock the PM 3) order more gin 4) become the longest serving monarch in British history #Busy— Elizabeth Windsor (@Queen_UK) September 6, 2015
This article first appeared on PR Week.