Blue laws, also known as Sunday laws, Sunday trade laws, and Sunday closing laws, are laws restricting or banning certain activities on specified days, usually Sundays in the western world. The laws were adopted originally for religious reasons, specifically to promote the observance of the Christian day of worship, but since then have come to serve secular purposes as well.

Sunday is the day of the week between Saturday and Monday. Sunday is a day of rest in most Western countries and a part of the weekend. In some Middle Eastern countries, Sunday is a weekday.