St Andrew is the patron saint of Scotland (and of Russia and Romania) and his feast day is celebrated by both eastern and western Christian Churches on 30 November. This is also Scotland's national day, though debate continues in Scotland about whether St Andrew's day should be a public holiday: currently it isn't.
St Andrew was one of the 12 Apostles, and brother of St Peter. In Greek he is known as Andreas and in the Orthodox Church is referred to as Protocletos ("first called").
After the death of Jesus, Andrew preached across the lands surrounding the Black Sea and is traditionally believed to have been the first Bishop of Byzantium. He is also believed to have been crucified at Patras in Greece on an X-shaped cross which has since come to be called St Andrew's cross and which forms the basis of the Flag of Scotland, the Saltire. His preaching helps explain why he became the patron saint of Russia and of Romania: but why Scotland?
The legend is that some time around 750, St Andrew's relics were brought by St Regulus by sea from Constantinople to the settlement now known as St Andrews in Fife. At the time Fife was a Pictish Kingdom under the control of King Angus Macfergus.
There's a problem with this story, because the only St Regulus or St Rule on record was one of St Columba's companions, who lived two hundred years before St Andrew's relics were meant to have arrived in Scotland.
An alternative view is that St Andrew's relics were brought to St Andrews by St Acca, who had been Bishop of Hexham until his exile in 732. On this interpretation, St Regulus's involvement was probably invoked (ie invented) to make it appear to early Christians that St Andrew's relics arrived in St Andrews much earlier than they actually did.
And it probably helped to have an implied link back to St Columba himself in the story during a time when many in the Church believed that St Columba and not St Andrew should be the Patron Saint of Scotland. The outcome of this debate is clear from the inclusion of a page about St Andrew, a man who during his life never came within thousands of miles of Scotland, on this site. Opinions differ as to precisely how St Andrew came to fill the role, but it seems likely to have been cemeted by the influence of Saint Margaret of Scotland, who married King Malcolm III in 1070. She did much to promote the Roman Catholic church in Scotland at the expense of the Celtic Church, and St Andrew was very popular in her Hungarian homeland.
But history has always been malleable. Which is doubtless why the Declaration of Arbroath in 1320 stated that Scotland had been converted to Christianity by St Andrew, so allegedly proving that God would have wanted to preserve Scotland's independence from England.