St George's Day is celebrated by the several nations, kingdoms, countries, and cities of which Saint George is the patron saint. St George's Day is also England's National Day.
Most countries which observe St George's Day celebrate it on 23 April, the traditionally accepted date of Saint George's death in 303 AD.
St George's Day was a major feast and national holiday in England on a par with Christmas from the early 15th century. However, this tradition had waned by the end of the 18th century after the union of England and Scotland. http://en.wikipedia.org/wiki/St_George's_Day
New appointments to the Order of the Garter are always announced on St George's Day, 23 April, Saint George being the patron saint of England. http://en.wikipedia.org/wiki/Garter_Day