The United Kingdom (UK) is a monarchy ruled by Queen Elizabeth II, consisting of four countries: England, Northern Ireland, Scotland, and Wales. Formerly a member of the European Union, the UK had a referendum in 2016 to leave the EU, referred to as 'Brexit'.
The UK doesn't have a national holiday, as they're one of the only countries in the world that has not become independent from another nation, as most nations gained their independence from the UK.
The flag of the United Kingdom, known as the Union Jack, is a combination of the flags representing the patron saints of three of the four countries making up the UK. The blue and white is for Saint Andrew of Scotland, the diagonal red cross for Saint Patrick of Ireland, and the red cross for Saint George of England.
"United Kingdom" refers to the fact that the four countries within it are united into one kingdom ruled by a monarch. The name of "Great Britain" was used since medieval times when it was used to differentiate the island from Brittany in France, which was referred to as "Little Britain". Ireland, as with the independent nation of Ireland, is named after the matron goddess of Ireland, Eriu.