This is
Victoria Day, celebrated the Monday before May 25, a national holiday observed in honor of 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.
An amendment to the Statutes of Canada in 1952 established the celebration of Victoria Day on the Monday preceding May 25.
So, your friendly neighbors to the south wish you Happy Holiday!!