Why are most National Holidays on a Monday?

Understatement
By Andrew Davidson
on February 17, 2010

Enacted in 1971, Congress passed the Uniform Monday Holiday Act, which moved almost all national holidays to the closest Monday. This current short list includes Washington's Birthday (popularly-but unofficially-known as Presidents' Day), Memorial Day, Columbus Day and Martin Luther King Day.

This was done in order to ensure federal employees would be guaranteed some 3 day weekends. Interesting factoid!