What was celebrated for only the second time in British royal history by the current queen in 2012?

In 2012, Queen Elizabeth II celebrated her Diamond Jubilee, marking the 60th anniversary of her accession to the throne. This event was only the second time in British royal history that a monarch had reached such a milestone, the first being Queen Victoria in 1897. The jubilee featured various public celebrations, including a grand pageant on the River Thames and nationwide street parties.