Victoria Day is a federal Canadian public holiday

Public holidays in Canada

Public holidays in Canada known as "statutory holidays," "stats" or "stat holidays" are legislated at the national, provincial and territorial levels...

 celebrated on the last Monday before May 25, in honour of Queen Victoria's birthday. The date is also, simultaneously, that on which the current reigning Canadian sovereign's official birthday is recognized. It is sometimes informally considered as marking the beginning of the summer season in Canada.

The holiday has been observed since before Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 was formed, originally falling on the sovereign's actual birthday, and continues to be celebrated in various fashions across the country on the fixed date.