America has become a 24-hour shopping mecca, year round. About the ONLY days stores are closed is Christmas and Thanksgiving. However, you'll find even this is slowly changing. Memorial Day, rather than a time of reflection and remembrance, has become an excuse for huge store clearances, discounts and promotions. In such a large consumerism society, usually the money wins out. Having said all of this, due to the vastness of the nation, there are 'cultural' differences that very by region. In the 'South' which typically means the south-east part of the United States, where there is a larger number of Protestant church-goers, older traditions may prevail. On the west coast, such as California (Los Angeles in particular), virtually everything is open 24/7. In the Northeast, where there is a balanced mix of Catholic/Judaic mixes, many businesses have policies as to opening/closings.
I hope this helped