Business day. As used in this chapter with respect to banks, business day means that day, as normally communicated to its depository customers, on which a bank routinely posts a particular transaction to its customer's account.
31 CFR § 1010.100
When used in this chapter and in forms prescribed under this chapter, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this subpart. Terms applicable to a particular type of financial institution or specific part or subpart of this chapter are located in that part or subpart. Terms may have different meanings in different parts or subparts.