St Andrew was officially named the patron saint of Scotland in 1320. It's St Andrew's Day on the 30 November. He is the patron saint of Scotland. He is also the patron saint of Romania, Greece, Russia, Ukraine and Poland. Patron saints are chosen to be special protectors or guardians over things.
Andrew was present at the Last Supper and went on to preach Christianity in the provinces of Scythia, Achaea and Thrace and along the Black Sea and the Dnieper River, hence his adoption as the patron saint of Greece, Cyprus, Romania, Ukraine and Russia in addition to Scotland.
The day of the 30th November to commemorate St Andrew was actually started by a group of expats from the USA. These expats wanted a day to connect with their roots in Scotland and also remember St Andrew and celebrate all that he stood for and his contribution to Scottish society.