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.
Find out more at Canada.ca