Verified answer

In Canada, federal legislation was passed to make it the Monday before the 24th. Believe me, I think all holidays should be celebrated on their proper day, even when it doesn't result in a long weekend. Just be lucky they haven't changed Christmas day!

http://en.wikipedia.org/wiki/May_long_weekend

Because it has to be on the closest Monday before the 24th, if the 24th of May isn't a Monday itself.

so we can have a long weekend