A business day means any day except any Saturday, any Sunday, or any day which is a legal holiday or any day on which banking institutions are authorized or required by law or other governmental action to close.

The definition of a business day varies by region. It depends on the local workweek which is dictated by local customs, religions, and business operations. For example, in the United States and much of the Western world, they are typically Monday through Friday from 9 a.m. to 5 p.m. By contrast, for many eastern countries such as Japan, the normal business day is Monday through Friday from 8:30 a.m. to 7 p.m.

The length of a business day varies by era, by region, by industry, and by company. Prevalent norms have included the 8-hour day and the 10-hour day, but various lengths, from 4 to 16 hours, have been normal in certain times and places.

Business days are commonly used by couriers when determining the arrival date of a package. If a courier ships a parcel on a Thursday that will be delivered in "two business days", it will arrive on the following Monday if neither Friday nor Monday are holidays.

In finance, how business days are defined are called business day conventions and determine how payments are settled on contracts such as interest rate swaps.