The Google UK home page today is showing a picture of bagpipes, as November 30th is St Andrew's Day, St Andrew being the patron saint of Scotland.
The flag of St Andrew, a diagonal white cross on a blue background, is called the Saltire. Saltire is a term used in heraldry to mean a diagonally shaped cross, and comes from the Old French saultoir, meaning "stirrup cord" or "stile".
Wikipedia states that St Andrew's Day is an official flag day in Scotland, meaning that the Saltire should fly on all Scottish government buildings. The expression "flag day" is not generally used in Britain with this sense; flag day means something else entirely here: "a day on which money is collected for a charity in the street and donors are given small paper badges to show they have contributed" (Oxford Dictionary of English definition).
Wikipedia is misleading on this point. The Scottish government revised its flag-flying policy in 2005 and its instructions are that the Saltire should be flown every day. That's another reason why the expression "flag day" is rarely seen in Britain in the US sense - there isn't one single flag day. There are 18 established days in the UK as a whole when government buildings are obliged to fly the flag (see the dates and reasons here).