A blue law is (predominantly in US English) a term for a law whose purpose is to enforce religious standards. Most commonly they are restrictions on what can be done on certain days of the week, such as preventing liquor stores or car dealerships from selling their products on Sunday. While many have been repealed or declared unconstitutional, some such laws still remain.
- Greek shopkeepers stage strike on first day of Sunday trading
- French prime minister puts Sunday trading on list of economic plans
Christians who celebrate Saturday as their holy day of the week, such as Seventh Day Adventists, consider Sunday laws to be a form of persecution. Some adherents, following the writings of Ellen G. White, claim that a government mandated National Sunday Law will be a sign of the end times. 
Constitutionality in the US
Within the United States, Blue Laws have been found to be constitutional in the decision of McGowan v. Maryland because regardless of religious origins, the harm done is not religious harm but generally economic harm. So long as such laws can be supported by even the flimsiest secular rational they are said to not violate the first amendment.
While the term "Blue Law" is not in widespread use in the UK, the general principle of restricting activities on a Sunday is and has been a topic of public debate. In the 1990s Sunday trading rules were relaxed, in a move strongly opposed at the time by church groups. The present situation is that most large stores (over a certain floor area of retail space) are permitted to open for only six hours on Sunday, commonly choosing to do so from 10am to 4pm. Smaller shops are permitted to set their own hours, but in less busy towns and villages they will often have highly restricted hours or choose to close altogether.