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The Sovereign's birthday has been celebrated in Canada since the reign of Queen Victoria (1837-1901).  
 
May 24, Queen Victoria's birthday, was declared a holiday by the Legislature of the Province of Canada in 1845.  
 
After Confederation, the Queen's birthday was celebrated every year on May 24 unless that date was a Sunday, in which case a proclamation was issued providing for the celebration on May 25.  
 
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 birthday of King Edward VII, who was born on November 9, was by yearly proclamation during his reign (1901-1910) celebrated on Victoria Day.  
 
It was not an innovation to celebrate the birthday of the reigning sovereign on the anniversary of the birth of a predecessor.  
 
In Great Britain, the birthdays of George IV (1820-1830) and William IV (1830-1837) were celebrated on June 4, birthday of George III (1760-1820).  
 
The birthday of King George V, who reigned from 1910 to 1935, was celebrated on the actual date, June 3 or, when that was a Sunday, by proclamation on June 4.  
 
The one birthday of King Edward VIII, who reigned in 1936, was also celebrated on the actual date, June 23.  
 
King George VI's birthday, which fell on December 14, was officially celebrated in the United Kingdom on a Thursday early in June.  
 
Up to 1947 Canada proclaimed the same day but in 1948 and further years settled on the Monday of the week in which the United Kingdom celebration took place. George VI reigned from 1936 to 1952.  
 
The first birthday of Queen Elizabeth II, in 1952, was also celebrated in June.  
Meanwhile, Canada continued to observe Victoria Day. An amendment to the Statutes of Canada in 1952 established the celebration of Victoria Day on the Monday preceding May 25.  
 
From 1953 to 1956, the Queen's birthday was celebrated in Canada on Victoria Day, by proclamation of the Governor General, with Her Majesty's approval. In 1957, Victoria Day was permanently appointed as the Queen's birthday in Canada.  
 
In the United Kingdom, the Queen's birthday is celebrated in June.  
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.  
 
Physical arrangements means the existence of at least two flag poles: the Canadian flag always takes precedence and is never replaced by the Union Jack.  
 
Where only one pole exists, no special steps should be taken to erect an additional pole to fly the Union Jack for this special day.  
 
Source: http://www.pch.gc.ca/progs/cpsc-ccsp/jfa-ha/victoria_e.cfm