A day commerce runs — usually a synonym for working day, and not always.
Business day is the term contracts reach for when counting time. In most usage it is interchangeable with working day. Where it is not, it usually means a day on which banks and markets operate, which can exclude days an ordinary office is perfectly busy.
It is almost always defined by the instrument using it, and those definitions rarely match. One agreement may count a day the banks are shut and the offices open; another may not; a third may name specific cities and require all of them.
The safe assumption is that there is no safe assumption. Where a contract turns on the count, the definition is worth reading rather than inferring.
Frequently synonyms. Where they differ, business day tends to follow banks and markets rather than offices.
A banking day is specifically about banks being open. A business day may be broader.
See also working day · banking day · clear days · trading day
We hold this as derived — the corpus records closures, not contract definitions. See what we hold.