Q. 61. What is forbidden in the fourth commandment?
A. The fourth commandment forbiddeth the omission or careless performance of the duties required, and the profaning the day by idleness, or doing that which is in itself sinful, or by unnecessary thoughts, words or works, about our worldly employments or recreations.
— Westminster Shorter Catechism
What? It says we are supposed to refrain from unnecessary work so we can rest on that day.