Monday, May 23 is Victoria Day in Canada. This celebrates Queen Victoria's birthday (May 24th). Canada is still a member of the Commonwealth of Nations, of which the Queen is head.
Victoria Day is always on a Monday, and thus the holiday is part of a long weekend, which is commonly referred to as the Victoria Day Weekend, the May Long Weekend, the May Long, or the May Two-Four (a case of beer there is called a "two-four" and many of these are consumed over the holiday).
The Victoria Day Weekend is the first popular weekend for spring / summer travel. Lots of people open up their cottages, plant gardens, or just get away. Gorgeous fireworks displays are common, especially on Monday night. If you are lucky enough to join in the festivities, expect crowds at resorts and hotels and very busy highways.
Banks, schools, many stores and restaurants are closed on the Monday, so be sure to call ahead to find out about attractions and tourist spots that you plan to visit. Happy "two - four" Canadian friends!