A blue law is one of numerous extremely rigorous laws created and designed to regulate the morals and conduct in early colonial New England. More generically it is also a statute that regulates work, commerce, and amusements on Sundays.

The first known evidence of a blue law comes from the Israelites idea of taking a day of rest which was a concept completely foreign to the Egyptians, one of the many causes of conflict between the Jewish slaves and their masters; one more reason they fled their oppressors. It comes from the Fourth Commandment written by God and given to Moses in the Ten Commandments in Exodus:

Remember the Sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work: But the seventh day is the Sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the Sabbath day, and hallowed it.

Furthered by Constantine's Sunday Blue Law given the seventh of March by consuls Crispus and Constantine in 321 A.D.

"Let all judges and all city people and all tradesmen rest upon the venerable day of the sun. But let those dwelling in the country freely and with full liberty attend to the culture of their fields; since it frequently happens that no other day is so fit for the sowing of grain, or the planting of vines; hence, the favorable time should not be allowed to pass, lest the provisions of heaven be lost."

The Blue Law is so called because it was written on blue paper when enacted by Puritan colonies in the 1794 in Colonial Pennsylvania and were passed to enforce certain moral standards and particularly to prohibit specified forms of entertainment or recreation on Sundays such as limiting sales of various products, dancing drinking and working on Sunday. Some have been repealed or modified in most parts of the United States. Blue Law prohibited the operation of businesses and made Sunday a mandatory day of spiritual observance. This came into conflict with other religions specifically Seventh Day Sabbath who observe Saturday as the Sabbath and forced them to abstain from any business practice two days out of every week. As the number of states imposing Blue Law restrictions began to spread throughout the eastern United States, a definite pattern of westward migration by Seventh Day Sabbath keepers occurred during the early decades of the 19th century.

Both Fourteenth and Fourth Amendment issues regarding blue laws in the United States still come into conflict as a hindrance with many business practices today.

The precedent for blue law is the concept present at the heart of sumptuary law from antiquity.