Definition of blue law noun from the Oxford Advanced Learner's Dictionary

 

blue law

 noun
noun
BrE
 
; NAmE
 
[usually plural]
 
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(in the US) a law that bans business and certain other activities, such as sports, on Sundays Culture Blue laws were first introduced in colonial New England and were originally printed on blue paper. Now they vary widely in different parts of the US. Although many shops open on Sundays, they often do not sell alcohol on that day because of a blue law.
See the Oxford Advanced American Dictionary entry: blue law