Business day.
Business day. Any day, other than a Saturday, a Sunday, or a legal holiday (which includes any holiday in the District of Columbia and any statewide holiday in the particular State in which the claim, report, or return, as the case may be, is required to be filed, or the act is required to be performed).
Source
27 CFR § 19.1
Scoping language
As used in this part, the following terms shall have the meanings indicated unless either the context in which they are used requires a different meaning, or a different definition is prescribed for a particular subpart, section, or portion of this part: