Victoria Day (National Patriotes Day in Quebec) is Canada's oldest non-religious holiday. It honours Queen Victoria's birthday.
When Queen Elizabeth II ascended to the throne in 1952, Victoria Day became her official birthday in Canada -- even though her actual birthday is April 21. After the death of Queen Victoria in 1901, an Act was passed by the Parliament of Canada establishing a legal holiday on May 24 in each year (or May 25 if May 24 fell on a Sunday) under the name Victoria Day.
The Royal Union Flag, commonly known as the "Union Jack" where physical arrangements allow, is flown along with the National Flag at federal buildings, airports, military bases and other federal buildings and establishments within Canada, from sunrise to sunset, to mark this day.