St. George's Day is on the 23rd April. It is England's National Day.
St. George is the Patron Saint of England. His emblem, a red cross on a white background, is the flag of England and part of the British flag.
St. George was a brave Roman soldier who did not like how the Romans' treated Christians.
The picture you see of St. George slaying a dragon is to show how he stood up for all Christians against the Romans and died for his beliefs.