(CNN)Victoria Day will be celebrated across Canada on Monday, May 18.
The holiday began as a celebration to honor Queen Victoria, who ruled Great Britain and Ireland for most of the 19th century.
Victoria Day has been observed since 1845, and it was officially declared a Canadian federal holiday in 1901 -- the year of Queen Victoria's death.
The holiday was originally celebrated on May 24, Queen Victoria's birthday. But an amendment to the laws of Canada in 1952 established that Victoria Day be observed on the Monday preceding May 25.
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.
The day unofficially signals the start of summer for many Canadians, and it is celebrated with parades and fireworks in some cities. Some Canadians get the day off from work.
The Royal Union flag is flown alongside the National flag at federal buildings, airports and military bases from sunrise to sunset to mark the occasion.