The first Father's Day was observed on June 19, 1910 in Spokane, Washington.
During WWII, Father's Day became more widely observed, and in 1966, President Lyndon B. Johnson declared the third Sunday of June to be Father's Day. President Richard Nixon made it a federal holiday in 1972.
The word "Dad" has been used for hundreds of years... possibly as far back as the sixteenth century.
Why is it spelled Father's Day and not Fathers' Day? Having the apostrophe before the "s" makes it a celebration of your very own dad, and not to all fathers collectively.