A more thorough explanation:

Definition: A Sunday law, also known as a blue law, is a statute that regulates or prohibits commercial activity on Sundays. These laws were once common but have declined since the 1980s due to their religious origins. Sunday is considered the Christian Sabbath, and blue laws were enacted to observe this day of rest. However, many courts have held them invalid because of their religious origins. Blue laws can pass constitutional challenge if they are enacted to support a nonreligious purpose, such as a day of rest for workers.

Examples: A Sunday law may prohibit businesses from opening on Sundays or restrict the sale of certain items on that day. For example, in some states, car dealerships are not allowed to sell cars on Sundays. In other states, alcohol sales may be restricted on Sundays. These examples illustrate how Sunday laws regulate commercial activity on a specific day of the week.