The departure from an "authentic" date is because some government ex-spurt decided that all holidays should come on Mondays.

The ex-spurt figured that eliminating breaks in the middle of the week would cause less distruption in the efficient operation of government offices.

Citizens with minimal awareness of government operations liked the idea, but on the supposition that "nobody in the government does anything on Mondays anyway."

Knowledgeable citizens didn't like the idea all that much, since keeping government workers out of the office as frequently as possible sounded like a good idea, and a week with "two Mondays" was about as close as you could get without adding more holidays.

The "Monday holidays" in the US are uniformly observed by government offices/agencies, including most state and local government offices, and banks. Some businesses may recognize one or two of them, but shutting down isn't mandatory for businesses.

John