A strange phenomenon has happened with holidays. It no longer matters when the actual meaningful day once was, all we really care about is having continuous time off. In other words, the holiday gets moved to a nearby Monday (or, less often, Friday) in order to accommodate The Long Weekend.
The Long Weekend sometimes seems like an unusual concept in and of itself. Whole countries decide 'Hey, we have an extra day, let's go do...' whatever it is they particularly want to do. It doesn't matter that everyone else has the same idea, that the roads are jammed, and wherever they're headed will be more crowded than any other time, they just cannot pass up this chance to have a day out on this particular day.
Some examples of oft-misunderstood three-day weekends:
Bank Holidays (UK): To people living outside the UK, Bank Holidays possess an air of mystery. The elusiveness of the term, the mysterious correspondence with other long weekends across the globe... is there something we're not being told?
Remembrance Sunday (EU, USA) or Veteran's Day: November 11 marks the Armistice at the end of WW1, but it now officially recognises both World War I and World War II. At one point November 11 was generally called 'Armistice Day'. The UK now celebrates it on the nearest Sunday and calls it 'Remembrance Sunday', and in the USA, it is observed on the 11th and known as 'Veteran's Day'.
Memorial Day (USA): Most people seem to assume that it is for honouring war veterans and throwing fantastic barbecues. In fact, the first official observance of Memorial Day was instigated by Civil War General John A. Logan, who ordered all men in his command to honour the dead by putting flowers on grave sites on the 30th of May. The tradition was later modified to be celebrated the last Monday in May. This is also the 'Spring Bank Holiday' in the UK. Ireland's mysterious 'June Bank Holiday' happens the following Monday.
Labor Day (USA): Labor Day is held in honour of the working class. It began with a large parade thrown by the Knights of Labor in New York City in 1882. The parade was such a smashing success that it was determined by the group that the same parade should take place every first Monday in September. In 1894, the U.S. Congress declared it a legal holiday.
Note that Memorial Day and Labor Day bracket the summer months rather nicely, making them even more popular. It is best to avoid travel to those favourite destinations of theme parks and beaches on these weekends, unless you like to be there with thousands and thousands of your 'closest friends' or, more than likely, get stuck in a stationary queue of traffic.